Not really rule of 5

If we know an integer is a multiple of 5, how many possibilities are there for the last two digits of the integer?


The answer is 20.

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13 solutions

Cyril Bonganay
Sep 15, 2013

For a number to be divisible by 5, the last digit must be 0 or 5. Thus we have two options.

For the digit before the last digit, we can choose from 0 to 9. As long as the last digit is 0 or 5. We have 10 options...

Therefore

10 × \times 2 = 20 possibilities

Kudos for using Latex to type up the math.

As a note, you needed type it as "\times" in order for it to display as × \times . Typing "times" simply gives t i m e s times . In a sense, the backslash is code for "Change the upcoming command into a mathematical symbol". I've edited your equation so that it displays properly.

Calvin Lin Staff - 7 years, 8 months ago

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Thank you for the info. :)

Cyril Bonganay - 6 years, 11 months ago

This is like an Arithmetic Progression.

The sets of numbers are:

00, 05, 10, 15 and so on till 95 because what we need are the las two digits.

Hence:

a 1 a_{1} which is the first term of the AP = 0

a n a_{n} which is the last term of the AP = 95 = a 1 a_{1} + ( n 1 ) × d (n-1) \times d where d is the common difference of 5.

Implies:

95 = 0 + (n-1)d = ( n 1 ) × 5 (n-1) \times 5

95 5 \frac{95}{5} = n-1

19 = n-1

n =20 = No. of Possibilties

Too much proness.

Ronq Vader - 7 years, 3 months ago

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Respect 2u

Jacob Reuben - 3 years, 5 months ago

Super thinking pattern

Vinuka Karunaratne - 3 years, 2 months ago

Nicely done, but could be done simpler.

N K - 2 years, 7 months ago
Anzar Aznzar
Mar 30, 2014

100/5 = 20

Jasmeet Sharma
Mar 18, 2015

see we have to fill 2 places _ _
we can fill the first place means the tens place with 10 digits ( from 0 to 9) So, we have 10 possibilities to fill the 10's place.

And we can fill the ones place only with 0 or 5 as according to the divisibility rule of 5 ones place must be 0 or 5 . so we have only 2 possibilities for ones place.

so total possibilities are 10x2=20

Jamonte Grant
Sep 20, 2013

00, 05,10,15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95= 20 Possibilities

Sd Turja
Sep 16, 2013

The last number of the multiple of 5 must be 0 or 5. we have 0 to 9 number. We can take it 10 times.. 10times2=20 possibilities

If the number is divisible by 5 the last digit should be 0 or 5. Anyway they ask for the last 2 digits.There are 10 one digit numbers (0,1,2,3,4,5,6,7,8,9).All these numbers can be the one before the last digit.All the numbers can go with 2 possible last digits namely 0 & 5.

       So,
           ✳ 10 ✖ 2 = 20
Vikas Chaudhary
Sep 20, 2013

at second last digit 10 options at last only 2 as 0 and 5 so total 10*2=20

David L.
Sep 19, 2013

The divisibility test of 5 is whether it ends with 0 or 5.If it ends with 0 or 5, it has 5 as a factor. For the second last digit, it can be any digit (0,1,2,3,4,5,6,7,8,9). There are 2 cases for the last digit and 10 cases for the second last digit,so 2 × 10 2 \times 10 =10= the answer.

QED

Daniel Chiu
Sep 18, 2013

The number could end with 0 or 5. The second-to-last digit doesn't affect divisibility by 5, since 10 is a multiple of 5, so it can be any of the 10 digits. The answer is 2 10 = 20 2\cdot 10=\boxed{20} .

Kristan Liza
Sep 18, 2013

If the integer is divisible by 5, then the units digit must be 0 or 5. If we are looking for the last two digits of an integer that is divisible by 5, then the tens digit can any be number from 0 to 9. Therefore, the total number of possibilities is 10 x 2 which is 20.

Jeremy Lu
Sep 17, 2013

All multiples of 5 must end in 0, or 5. The second to last digit can be anything. So we have 10*2=20

Priyansh Sangule
Sep 17, 2013

We know that a number is divisible by 5 5 iff - the last digit is either 0 0 or 5 5 .

Therefore we have to choose for two spaces :

A B \boxed{A} \boxed{B}

Now for box A : We have 10 10 possibilities : 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 {0,1,2,3,4,5,6,7,8,9}

And for box B : We have 2 2 possibilities : 0 , 5 { 0 , 5 }

Thus possibilities can be written as :

10 2 \boxed{10} \boxed{2}

Therefore total number of possibilities are : 10 2 = 120 10 \cdot 2 = \boxed{120}

Typo in the last answer - 20 \boxed{20}

Priyansh Sangule - 7 years, 8 months ago

Lame error- 10 × 2 = 20 10\times2 = 20 and not 120 120 . :P

Akshat Jain - 7 years, 8 months ago

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:)

sankar narayanan - 7 years, 8 months ago

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