How old is Amy?

John and Amy have the mathematical conversation above. How old is Amy?


The answer is 23.

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12 solutions

Eli Ross Staff
Sep 18, 2015

The sum of the digits must be prime, so one digit must be odd and one digit must be even (since the sum of the digits cannot be 2, since the digits of 11 are not prime). Since 2 is the only even prime, one of the digits is 2.

Furthermore, 2 must be the first digit, otherwise the number would be even (and thus not prime). Therefore the options are:

23, which works, since 2 is prime, 3 is prime, 2 + 3 = 5 2+3=5 is prime, and 23 is prime

25, which itself is not prime

27, which itself is not prime (nor is the sum of its digits)

Thus, Amy is 23 years old.

amy cannot be that old.. 32, 43 etc., as the looks of the girl cannot be more than 11. 11 is the answer. Moreorver, both the children are in schooling stage.

Gold Sugar - 4 years, 5 months ago

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This is more like a cartoon than a photo when it comes to pictures(we should work this out with maths anyway).

Zoe Codrington - 2 years, 7 months ago

could it be 32

safia ali - 5 years, 5 months ago

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32 is not prime.

Panya Chunnanonda - 5 years, 5 months ago

Why not 25? 2 is prime, 5 is prime and the sum of they is prime(7)

Anderson Felipe Aedo - 5 years, 2 months ago

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Because 25 is not prime.

Burcu Tava - 5 years, 2 months ago

25 is not a prime number

Katie Young - 5 years, 2 months ago

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they only asked the digits not the number itself

I Love Brilliant - 10 months, 2 weeks ago

25 is not a prime number

majd khoury - 5 years, 1 month ago

It never says the number itself has to be prime, therefore there are many answers. 32 could work just as well. 92 can work if 29 . Conditions: 1. sum of digits must be prime. 2. Each digit must be prime. Therefore this number is set automatically between 22 and up but must contain a 2.

Wellington Chen - 4 years, 9 months ago

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the girl says "well its a PRIME but what makes it so super"

Liam McCollum - 4 years, 6 months ago

He should say "both it's digits are prime as is the sum of it's digits AND the number itself", otherwise there are a lot of possible combinations.

Rui Marques - 4 years, 6 months ago

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True but she said, "Well it's prime ...

Leigh Thompson - 3 years, 2 months ago

Agree. Not well written.

H Colon - 1 year, 2 months ago

The condition given in the question is to find out 2 digit prime where sum of the digits will be prime also.. So, the required answer will include 23=2+3=\color\red\boxed{5} .. But the number 32 32 can be a solution. As 32 32 is not prime, the answer is 23 23

Prokash Shakkhar - 4 years, 6 months ago

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The girl says'Well it's a prime, but what makes it super?'

Zoe Codrington - 2 years, 7 months ago

23 is not a super prime number

Bob Butler - 4 years, 1 month ago

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Re read the question

Zoe Codrington - 2 years, 9 months ago

Haha that makes sense. I forgot about the number having to be prime itself and was screaming at my phone how 25, 32, and 52 were not also superprime numbers and how 23 was the only one that worked. Thanks for the explanation.

Dani Watson - 3 years, 11 months ago

21 is also solution!

Irina Dribinskaya - 3 years, 7 months ago

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1 is not prime. A prime number is a whole number with only two distinct factors, 1 and itself.

Leigh Thompson - 3 years, 2 months ago

11, 12, 21 ??

Sameer Pai - 3 years ago

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1 is not prime. A prime number is a whole number with only two distinct factors, while 1 has only one factor.

Hua Zhi Vee - 2 years, 12 months ago

it says the number is a superprime, and 23 is not, so technically no solution which satisfy all the condition, may be she's lying

vyshnav v - 2 years, 11 months ago

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why isn't 23 a superprime? It is!

Hua Zhi Vee - 2 years, 11 months ago

By the conditions given in the problem, 23 is a superprime.

Zoe Codrington - 2 years, 7 months ago

The actual definition of Super prime is not the one given in the problem. A super prime number is any prime number that occupies a prime number position in a numerical index. Say you have 5 spots numbered 1, 2, 3, 4, 5. You then fill those spots with the first prime numbers; 2, 3, 5, 7, 11. Because 3, 5, and 11 occupy the "prime" spots, they are super prime. Essentially the second, third, fifth, seventh, eleventh, etc etc prime numbers in order are super prime. So, Lets examine the conditions given in the problem. First off, forget the true definition of super prime. Since this problem neglected the true definition, we can only use the rules given in the statement. It states that the number is superprime, but never states that the number itself is a prime number. It then goes on to provide its own definition of super prime. "both digits are prime, and the sum of its digits are prime." So by the rules of the problem, the only conditions we need to meet are that both digits are prime and that the sum of both digits is prime. 23 and 32 work!

Jared Miller - 2 years, 7 months ago

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Thank you for the definition of superprime. :D

Amy said: "Well it's prime...", so I assumed that it means that superprime is prime.

Hua Zhi Vee - 2 years, 7 months ago

You are giving the definition of the Super prime. They are giving one of the superprime. There is a difference. Anyway, imagine this is the true definition for the problem, Amy says: "Well it's prime... but what makes it so super?". That is when the conditions you stated were made. However, the problem also implies that only one two digit number is a superprime(in fact, it is THE superprime) and Amy can only be one age. Even if it was not stated that it was prime, you can limit it down by using common sense(after all, they are referring to it as the superPRIME, implying it must be prime).

Zoe Codrington - 2 years, 7 months ago

52 and 25 are also super primes

Aditya Gupta - 2 years, 5 months ago

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52 and 25 are not primes\

Hua Zhi Vee - 2 years, 5 months ago

52 and 25 are not primes as 52 is even and 25 is divisible by 5.

Emily Peng - 1 year, 6 months ago

T H A T

M A C K E S

N O

S I N S E

Naven Oster - 1 year, 4 months ago

@Eli Ross , 23 23 is not a super-prime.

A Former Brilliant Member - 11 months, 4 weeks ago

Why not 11?

Aarushi Sood - 5 years, 7 months ago

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1 is not a prime no nor a composite no

Konark Gupta - 5 years, 6 months ago

1 is not considered to be a prime number.

See: https://primes.utm.edu/notes/faq/one.html

Raizel D - 5 years, 6 months ago

1 isn't considered prime because it fits the definition in a controversial way.

Zoe Codrington - 2 years, 9 months ago

What about 29? 29 is a prime, and so is 11

maria dm - 5 years ago

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9 is not prime

Kevin Merck - 5 years ago

1 is not a prime number.

Hua Zhi Vee - 5 years ago

What about 2 + 17 = 19

Erik Scheller - 4 years, 9 months ago

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17 is not a digit.

majd khoury - 4 years, 8 months ago

57 would work 5+7=13 = all prime digits

Paal Mangor-Jensen - 3 years, 11 months ago

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5+7 = 12.....

Jared Miller - 2 years, 7 months ago

Prank its 12

Paal Mangor-Jensen - 3 years, 11 months ago

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However 5+7=13 base 9, but 13 base 9 factors to 3 x 4 so 13 base 9 is not prime!

Leigh Thompson - 3 years, 2 months ago

Besides, 57 isn't prime

Zoe Codrington - 2 years, 9 months ago

It can also be 43

aryan tomar - 3 years, 8 months ago

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4 is not a prime number

Hua Zhi Vee - 3 years, 7 months ago

why not 43?

Ashee Khan - 5 years, 8 months ago

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4 is not a prime number

Yeasir Shourov - 5 years, 6 months ago

4 is composite.

Kenneth Gravamen - 5 years, 8 months ago

4 isn't prime. it's divisible by 2

Yash Srivastava - 5 years, 7 months ago

4 is Not prime

Graziano Calabresi - 5 years, 4 months ago

4 is not a prime number

safia ali - 5 years, 5 months ago

What about 25?? It too should work...

Mayank Kumar - 4 years, 3 months ago

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It's not a prime

aryan tomar - 3 years, 8 months ago

25 would work, too.

José Leite Júnior - 5 years, 8 months ago

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25 would not work. 2 is prime, 5 is prime, 7 is prime, but 25 is not. 5*5 is 25.

Travis Terwillegar - 5 years, 8 months ago
Samir Shaheen
Sep 19, 2015

11 also is a super prime.

1 is technically not a prime due to the factor of a prime number must be only multiplied by 1 and it self and since 1 is itself it fails to meet this requirement

Luke Walsh - 5 years, 8 months ago

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actually, 1 is not prime due to the fundamental theorem of arithmetic, which states that every natural number can be written as an unique product of prime numbers, and if 1 were prime, the factorizations wouldn't be unique, e.g. 36 = 2x2x3x3. if we assume 1 is prime 36 = 2x2x3x3 = 2x2x3x1 = 2x2x3x3x1x1 = 2x2x3x3x1x1x1...

André Winston - 5 years, 8 months ago

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Actually a prime must be divisible (not multiplied) by 1 and by itself. This is the rule for sure. But in the case of 1 the fact that it is divisible by 1 and it is divisible by itself (which by coincidence is again 1) does not disqualify the number by being prime. Moreover 23 is not the single answer what about 32? Isn't a super prime? But I understand that Amy prefers the first solution!

Luis Daselbe - 3 years, 7 months ago

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@Luis Daselbe 32 is not a prime number.

Steve Cau - 2 years, 8 months ago

@Luis Daselbe Luis, 32 is not prime, 32 is divisible by 1,2,4,8,16,32.

Weilian Gao - 2 years, 5 months ago

Also, prime numbers necessarily should have exactly two factors. 1 only has one factor so while it isn't composite, it's not prime either.

Julienne Bermejo - 3 years, 7 months ago

1 is not prime.

Wellington Chen - 4 years, 9 months ago

1 is not a prime. See more here: Is 1 prime?

Eli Ross Staff - 5 years, 6 months ago

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Technically, it could be.

Keegan S - 2 years, 8 months ago

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I know...

But I did some reaseach and it makes sense not to be. But I have to ask about the poor definition of units.(and compisites)

Zoe Codrington - 2 years, 8 months ago

i also picked 11 which is prime and so is the sum of its numbers

simon EDDISON - 4 years, 1 month ago

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Each digit must also be prime, and 1 is not prime

Zach Mara - 3 years, 9 months ago

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It a previous problem they show 1 as a prime number. So have we changed the definition,

Stanley Ziegler - 3 years, 8 months ago

23 is not a super prime number.

Sharon Black-MacKinnon - 3 years, 3 months ago

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2 is prime, 3 is prime, 23 is prime and 5 is prime.

Mitch Freeby - 3 years, 1 month ago

Really, at 23 years old they are having this conversation? I would have bet the number was 11. 1 is prime

Liviu Sabau - 2 years, 8 months ago

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It satisfies the definition, but it is not prime due to the fundamental theorem of arithmetic

Zoe Codrington - 2 years, 7 months ago

Do not forget that "1" is not a prime number

Nikolas Кraj - 2 years, 6 months ago
Christian Litster
Oct 26, 2016

I put 32, which technically works, but nope, it just HAS to be 23. What is this, AP Multiple Choice, we gotta pick the BEST answer, not one of the other correct answer??

Since when is 32 itself a prime?

Peter van der Linden - 4 years, 7 months ago

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Correct, I think question forgot to add that

Ruchira Moolwaney - 3 years, 7 months ago

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Well it's prime ... but what makes it so super?

The question does specify the age must be prime with this dialog.

Justin C - 2 years, 8 months ago

Since it is the sum of its digits and not the number itself...

Jorge Coronado - 4 years, 4 months ago

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My bad... I just read again lol

Jorge Coronado - 4 years, 4 months ago

The very first statement begins with John saying, "'My favorite number is the superprime!'" So the first statement tells you the number has to be prime. 32 doesn't work here, because its factors are 2 2 2 2 2. Only 23 works (1*23). Make sure you fully read the question!

Joseph Otoshi - 4 years, 1 month ago

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I didn't get from the question that the answer had to be prime

Harold Ship - 3 years, 10 months ago

can 2B(hex) be the answer? 2, 11 are prime 2+11=13 43(dec) is prime. :)

Ron Lee - 4 years ago

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I think it's cheating to use different bases

Zoe Codrington - 2 years, 9 months ago

But, 29 is prime and so is 11. Am I reading "sum of it's digits wrong?"

Devin Henkel-Legare - 4 years ago

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9 isn't prime

John Hayworth - 3 years, 8 months ago

Why doesn't 11 work? 11 is prime and 1+1=2, 2 also being a prime number.

Erik Ramsey - 3 years, 7 months ago

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Because 1 isn't a prime number. Remember, a prime number is a number that is divisible by 1 and itself. Since 1 can ONLY be evenly divisible by 1, it is technically not prime (1 is only divisible by one number, not two).

Joseph Otoshi - 3 years, 6 months ago

32 is divisible by two four eight and sixteen.

Philbert Teo - 3 years, 7 months ago

32 is not a prime no. bro

Deepak Bhatt - 3 years, 6 months ago

No, its the SAT! LOL

Alice Edwards - 3 years, 4 months ago

There is only one answer as both digits, their sum and the number must be prime and 32 is not prime. The number has to be prime as she said, "Well it's prime ... Your answer would be OK if only his comments were acted on in which case 23, 25, 32 and 52 would all be correct if you took superprime to mean only both digits and their sum must be prime. Super-prime numbers are prime but I'm not sure about superprime numbers. The two definitions of super-prime numbers are: Definition 1 Given p(1) = 2, p(2) = 3, p(3) = 5, p(4) = 7, p(5) = 11, p(6) = 13, p(7) = 17, p(8) = 19, p(9) = 23, p(10) = 29, p(11) = 31, p(12) = 37, p(13) = 41, p(14) = 43, ... then p(2), p(3), p(5), p(7), p(11), p(13), ... are super-prime i.e. 3, 5, 11, 17, 31, 41, ... are super-prime. Definition 2 A super-prime number is a prime number if when doubled less one is prime. e.g. 2 x 2 - 1 = 3, 2 x 3 - 1 = 5, 2 x 5 - 1 = 9, 2 x 7 - 1 = 13, 2 x 11 - 1 = 21, 2 x 13 - 1 = 25, 2 x 17 - 1 = 33, 2 x 19 - 1 = 37, ... i.e. 2, 3, 7, 19, 31, 37, 79, ... are super-prime. As 23 is not a super-prime number but is a superprime then as she said, "Well it's prime ... might be the only reason it must be prime.

Leigh Thompson - 3 years, 2 months ago

32 incorrect because 32 isn't a prime.

Haoran Wang - 2 years, 11 months ago

She looks more 23 than 32 in the picture. And 32 is not prime.

Liviu Sabau - 2 years, 8 months ago
K T
Mar 15, 2019

Let p = a b p =\overline{ab} be the super prime. The digits a and b must be prime, so digit candidates are 2, 3, 5, 7 The digit sum a+b (which is at least 4) must be prime too, so because b must be odd, a will be even, so a must be 2. Three candidates remain: 23, 25, 27 of which 23 is the only prime.

Jared Miller
Oct 31, 2018

The actual definition of Super prime is not the one given in the problem. A super prime number is any prime number that occupies a prime number position in a numerical index. Say you have 5 spots numbered 1, 2, 3, 4, 5. You then fill those spots with the first prime numbers; 2, 3, 5, 7, 11. Because 3, 5, and 11 occupy the "prime" spots, they are super prime. Essentially the second, third, fifth, seventh, eleventh, etc etc prime numbers in order are super prime. So, Lets examine the conditions given in the problem. First off, forget the true definition of super prime. Since this problem neglected the true definition, we can only use the rules given in the statement. It states that the number is superprime, but never states that the number itself is a prime number. It then goes on to provide its own definition of super prime. "both digits are prime, and the sum of its digits are prime." So by the rules of the problem, the only conditions we need to meet are that both digits are prime and that the sum of both digits is prime. 23 and 32 work!

Thomas Sutcliffe
Nov 20, 2017

The very first thing mentioned about the number is that is the two-digit superPRIME. Amy then says "well it's PRIME, but what's so super about it". John explains "It is PRIME, so are both its digits and so is the sum of both its digits". Thus we are looking for a PRIME number both of whose digits are themselves PRIME and which add together to make a prime. The only even prime is two, and one is not considered to be a prime, so the digits cannot add up to two. Thus, since the digits add up to an odd number (to be prime) two must be one of those digits, necessarily the first as otherwise the original number would not be prime. This gives us three theoretical possibilities for the second digit - 3, 5 and 7 are the only odd single digit primes. 25 = 5 x 5 and 27 = 3 x 3 x 3, so both of those are ruled out. Therefore 23, which is prime is correct - and a check reveals that 2 and 3 are both prime and 2 + 3 = 5 which is also prime. The reason for the capitalisation of the word Prime in the preamble to this solution is the sheer number of comments I have seen on other solutions that indicate failure to observe the actual requirements of the problem, so I thought I would emphasise them.

Yes, thanks. We needed that. People should read the question more. :)

Zoe Codrington - 2 years, 8 months ago

2 is a prime and 3 is a prime , their sum is 5 (also a prime)

it dose say right at the start "my favorite 2 digit number is a super prime"

Liam McCollum - 4 years, 3 months ago

why not 32 then?

Mick Martucci - 4 years, 5 months ago

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32 is not a prime, it says in the conversation that the age is prime number . . therefore it must be 23 . .

A Former Brilliant Member - 4 years, 5 months ago

Why is it the only one? You don't explain that.

Zoe Codrington - 2 years, 8 months ago

Not helpful at all

Naven Oster - 1 year, 4 months ago
. .
Jul 23, 2020

The first and second digit is all prime numbers. Then, the first prime number is 2. The second prime number is 3. So 23.

L Chua
May 12, 2020

i think this is a stupid question. there are a lot of possibilities.

the possibilities are:

-11 (provided 1 is prime)

  • 23

-32

-25

-52

therefore, there is NO concrete answer

Emily Peng
Nov 20, 2019

Amy has a two digit age. For two prime numbers to have a prime sum, there must be one even digit, as two odd numbers is even, and even numbers are never prime (aside from 2). Since 2 is the only even prime, it must be in the number. It must also be in the ten's place since otherwise, the number is even. 23 is the only odd number starting with 2, and it fits the description. Thus, Amy is 23.

Ervyn Manuyag
Nov 8, 2018

2 is prime, 3 is prime, and 2+3=5 which is also prime

Zoe Codrington
Nov 2, 2018

The problem claims that it is a two digit number that is the superprime.

Then Amy says"Well it's PRIME, but what makes it so super". This infers that the number is a prime. John then explains and lays down the conditions of a number being a superPRIME(excluding the one already mentioned, that the number itself is a prime) "Both its digits are prime as is the sum of its digits!". So the conditions are:

1.It is a two digit number

2.The number is prime(many people have missed this) .

3.Both it's digits are prime(One is not considered to be prime due to the fundamental theorem of arithmetic).

4.The sum of it's digits is prime

It is true that there is a real definition of superprime that is not the one mentioned in the problem, but we need to use these conditions for this problem.

Now, seeing as we are not meant to guess Amy's age using the picture, but rather use maths, we can then proceed:

One digit must be odd and the other even, as all primes but two are odd as a sum of two is impossible to obtain(11 is not a superprime. Check condition 3). Therefore two, the only even prime, is the even digit and also the first digit, to ensure the number is prime. The options are then as follows:

21.(Not prime)

  1. (Sastisfies all conditions.)

  2. (Not prime)

  3. (Not prime)

  4. (Digits not prime)

Therefore 23 is the answer.

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