Lucky remainder

How many positive integers a a are there such that 2027 divided by a a leaves a remainder of 7?


The answer is 8.

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27 solutions

Priyanka Banerjee
Oct 27, 2013

Since it has to leave a remainder of 7 thus is has to be divided by a number greater than 7..

We subtract 7 from the given number (2027-7=2020) and find all the factors of 2020 which are

1, 2, 4, 5, 10, 20, 101, 202, 404, 505, 1010, 2020

But we take only those >7 i.e. 10, 20, 101, 202, 404, 505, 1010, 2020

Thus there are 8 numbers which leave remainder 7.

did the same

Snehashis Mukherjee - 5 years, 1 month ago

did the same way

Ganesh Ayyappan - 5 years, 6 months ago

thanks a lot

Minh Nguyen Ngoc - 5 years, 6 months ago

Gud, first I answered 12 bt then in the second attempt I got it correct..!

PUSHPESH KUMAR - 5 years, 7 months ago
Finn Hulse
Mar 4, 2014

First, let us factorize 2027 7 2027-7 . This is 2 2 5 101 2^{2}*5*101 . Now we look for numbers that can be obtained from these numbers that are larger than 7. They are 20, 101, 10, 505, 202, 404, 2020, and 1010, which is 8 numbers.

Did you list out all the factors of 2020 2020 to solve this? If you did, there's a better way.

As 2020 = 2 2 × 5 × 101 2020=2^2\times5\times 101 , 2020 2020 has ( 2 + 1 ) ( 1 + 1 ) ( 1 + 1 ) = 12 (2+1)(1+1)(1+1)=12 divisors and four of them is less than 7 7 [they are 1 1 , 2 2 , 4 4 and 5 5 ].

It's easier to list out smaller divisors in my opinion.

Mursalin Habib - 7 years, 2 months ago

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I have solved it in your way. Its the easiest

Tahsin Ahmed - 7 years, 2 months ago

Yeah, I know the trick, and that's exactly what I used to solve the problem, but I just thought it would be better to list them for the solution. By the way, are you following me?

Finn Hulse - 7 years, 2 months ago

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Strange. I thought I did. Oh well, it's never too late!

Mursalin Habib - 7 years, 2 months ago

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@Mursalin Habib :D

Finn Hulse - 7 years, 2 months ago
Rizky Dermawan
Aug 11, 2013

If 2027 divided by a, then leaves a remainder of 7... So, a must greater than 7 Then you can solve this with modulo, 2027 7 m o d a 2027 \equiv 7 \ mod a , hence 2020 0 m o d a 2020 \equiv 0 \ mod a from this, we know that a is divisor of 2020. there are 12 divisor of 2020. but 4 of them are less than 7. So, the possibilities of a is 8

Moderator note:

A common mistake made is to ignore the fact that a > 7 a > 7 , which is not explicitly stated.

Anyways....I think that this is the quickest solution.....

Also, try \equiv 7(pmod{a}) in place of simply going \equiv 7 mod a It looks like 2027 7 ( m o d a ) 2027 \equiv 7 (\bmod{a})

Anthony J. - 7 years, 10 months ago

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If you have solution that nicer than me... it's very good. Now, in brilliant, everyone can give our opinion/solution in every question.

Rizky Dermawan - 7 years, 10 months ago

That fact is true in my case.Anyways, awesome problem!

Raven Herd - 5 years, 11 months ago
Trevor B.
Oct 28, 2013

If a number K K divided into N N leaves a remainder of R R , then K K has two important characteristics: K N R K | N-R and K > R K>R .

For this problem, K = a K=a , N = 2027 N=2027 , and R = 7 R=7 . a 2027 7 a 2020 a | 2027-7 \Rightarrow a | 2020 and a > 7 a>7 . The factors of 2020 2020 are 1 1 , 2 2 , 4 4 , 5 5 , 10 10 , 20 20 , 101 101 , 202 202 , 404 404 , 505 505 , 1010 1010 , and 2020 2020 . 8 8 of these factors are greater than 7 7 , so the answer is 8 \boxed{8} .

Ahaan Rungta
Oct 28, 2013

This problem translates to the modular equation 2027 7 ( m o d a ) 2020 0 ( m o d a ) a 2020. 2027 \equiv 7 \pmod a \implies 2020 \equiv 0 \pmod a \implies a \mid 2020. So we want the number of positive integer divisors of 2020 2020 that are greater than 7 7 .

We know that 2020 = 2 2 × 5 × 101 2020 = 2^2 \times 5 \times 101 , so it has 12 12 positive integer divisors. But, of these, 1 1 , 2 2 , 4 4 , and 5 5 are bogus, as they are less than 7 7 , so the answer is 12 4 = 8 . 12 - 4 = \boxed {8}.

what s modular equation?

Edgar de Asis Jr. - 7 years, 7 months ago

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https://en.wikipedia.org/wiki/Congruence relation#Basic example

Jan J. - 7 years, 7 months ago

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while you asking us you better search google

noufa mathur - 7 years, 7 months ago

A correction: a 2020 a \mid 2020 not a 202 a\mid 202 .

A Former Brilliant Member - 7 years, 7 months ago
Sam Leo
Oct 28, 2013

For 2027 % a = 7 2027\%a = 7 , a a has to be divisible by 2027 7 = 2020 2027-7 = 2020 but not divisible by 2027

Divisors of 2020: 1, 2, 4, 5, 10, 20, 101, 202, 404, 505, 1010, 2020 Divisors of 2027 (prime no): 1, 2027

However, for a a to leave a remainder of 7 7 , a>7 Therefore, the positive integer a a can be 10, 20, 101, 202, 404, 505, 1010, 2020 , which gives 8 numbers.

Nice solution, but it is unnecessary to state that a ∤ 2027 , a \not\mid 2027, I believe. Also, you should say "2020 is divisible by a," not "a is divisible by 2020."

Michael Tang - 7 years, 7 months ago

I'm sorry i have a question that has nothing to do with this problem.Brilliant has changed.When i go at ''Practice''everything i see is shortcuts.But brilliant used to have a lot of useful informations about mathematics at ''Olympiad''.Where can i find these stuff now?

Lorenc Bushi - 7 years, 7 months ago

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I'm sorry just ignore this comment, i found what i was looking for.

Lorenc Bushi - 7 years, 7 months ago
Shaan Bhandarkar
Aug 12, 2013

If 2027 7 m o d n 2027\equiv\ 7\mod{n} , then we immediately know that n must be a divisor of 2020. The prime factorization of 2020 is 2 2 × 5 × 101 2^2\times5\times101 which makes the number of divisors equal to 3 × 2 × 2 3 \times 2 \times 2 or 12 divisors. However, we are not done. If we divide 2027 by 1, the remainder is 0. So, we have to subtract any divisors of 2020 which are less than 7. This includes 1,2,4, and 5. So, from the total of 12 divisors we subtract 4 divisors to receive 8 \boxed{8} as the answer.

How are you able to determine that there are 12 divisors of 2020?

Joe Aliyev - 7 years, 10 months ago

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A formula for finding the number of divisors of N N is as follows:

If N = A a B b C c . . . N = A^a*B^b*C^c... where A,B,C are distinct prime numbers and a,b,c are number greater than 0 0 , the number of factors of N N is ( a + 1 ) ( b + 1 ) ( c + 1 ) . . . (a+1)*(b+1)*(c+1)... .

I am sorry I didn't say this explcitly.

Shaan Bhandarkar - 7 years, 10 months ago

From division algorithm ,

Dividend = Divisor * Quotient + Remainder ( Divisor is strictly > Remainder and Remainder is strictly +ve)

Therefore ,

2027 = a * q + 7 ( where q = Quotient, some + ve integer )

2020 = a * q

Now ,

2020 = 1 * 2 * 2 * 5 * 101

and we also know that a>7.

Therefore , possilble values of a are

a = 10 , 20 , 101 , 202 , 404 , 505 , 1010 , 2020 .

So , a can have the above 8 + ve integral values.

Jan J.
Oct 28, 2013

We need a 2020 = 2 2 5 101 a \mid 2020 = 2^2 \cdot 5 \cdot 101 , the number 2020 2020 has 3 2 2 = 12 3 \cdot 2 \cdot 2 = 12 factors, but we need a > 7 a > 7 , hence we ignore 4 4 factors, namely 1 , 2 , 4 , 5 1,2,4,5 , so there are 12 4 = 8 12 - 4 = \boxed{8} such values of a a .

Sharky Kesa
Oct 28, 2013

2027-7=2020 2020=2^2x5^1x101^1 (2+1)(1+1)(1+1)=12 Factors 1, 2, 4 & 5 are under 7 so 12-4=8

Christopher Boo
Aug 14, 2013

All integers n n can be written in the form

n = a r + b n=ar+b

where b b is the remainder.

2027 = a r + 7 2027=ar+7

2020 = a r 2020=ar

The prime factorization of 2020 2020 is 2 2 × 5 × 101 2^2\times5\times101

So, the number of factors of 2020 2020 (which is the number of possible values of a a ) is ( 2 + 1 ) × ( 1 + 1 ) × ( 1 + 1 ) = 12 (2+1)\times(1+1)\times(1+1)=12

However, in this problem a > 7 a>7 , among the 12 possible values, 1 , 2 , 4 , 5 1, 2, 4, 5 are not acceptable.

Thus, the answer is 12 4 = 8 12-4=8

Robi Fajar Bahari
Aug 11, 2013

a > 7 a>7 . there is a number p ( p > 2027 ) (p>2027) such that a p + 7 = 2027 ap + 7 = 2027 , a p = 2020 ap=2020 . so a is a positif divisor of 2020 greater than 7, there are 8 numbers.

Arindam Sanyal
Apr 21, 2019

A simple C++ program can solve this

#include <iostream>
using namespace std;

int main() {
    int i,r,count=0;
    for(i=1;i<=2027;i++){
        r = 2027%i;
        if(r==7) count++;
    }
    cout<<count;
    return 0;
}

1
2
3
4
5
num = 2027
remainder = 7
x = [a for a in range(remainder,num) if num%a == remainder]
print x       
print len(x)

1
2
[10, 20, 101, 202, 404, 505, 1010, 2020]
8

Micha Shepher
Dec 14, 2017

len([ i for i in range(1,2027) if 2027%i == 7])

Vijayant Patel
Mar 9, 2017

First subtract 7 from 2027 which gives 2020. 2020= 2×2×5×101. Therefore total no of factors can be calculated using P&C =3×2×2=12, but 1,2,4&5 can't leave 7 as remainder so no. of factors of 2020 greater than 7 are 12-4=8

Qais Kawalit
Jun 10, 2016

I wrote a program for it and got it write, took less than 2 minutes lol

Hana Wehbi
May 13, 2016

an+7=2027, we need a>=8; there for, solve for n= 2020/a for:{1,2,3,4,5,6,7,8} as a values, we obtain n={ 2020, 1010,505,404,202,101,20,10}. (8 integers)

Nikitha K
Jun 14, 2015

After reading the problem you should realize that; in order for a remainder of 7 to result the divisor MUST be greater than 7. So now we should doing the work.

Step 1: Rewrite the words into an equation we can solve. Let x be the other variable a is multiplied by. So the equation would be ax + 7 =2027

Step 2: Simplify the equation. ax = 2027-7 ax = 2020

Step 3: Find the factors of 2020. One way to ensure you found all of the factors is by starting off with prime factorization. The prime factorization of 2020 is 2020 = 2 \times 2 \times 5 \times 101 = 2^{2} \times 5 \times 101

Step 4 (3.5): Now multiply these to find all the factors of 2020. 2020 = 1 \times 2 \times 4 \times 5 \times 10 \times 20 \times 101 \times 202 \times 404 \times 505 \times 1010 \times 2020

Step 5: Get rid of the factors less than 7 (reasoning in first paragraph). So the factors you result with are 10, 20, 101, 202, 404, 505, 1010, 2020

Step 6: Conclusion - find final answer. For the answer just count up the number of numbers in the number set. Which is \boxed{8}

Kevin H
Nov 2, 2013

Given the number 2027 we can simplify this into which numbers are factors of 2020. Since 2020 leaves 2027 with a remainder of 7. Factoring out 2020 we get 2 \times 2 \times 5 \times 101, using the factor rule we add one to each of there powers resulting in 3 \times 2 \times 2=12 Hence there are 12 factors. We then find these 12 factors and they are 1, 2, 4, 5, 10, 20, 101, 202, 404, 505, 1010, and 2020. Now 1,2,4,5 cannot possibly be factors since they are less than 7 therefore this leaves us with 8 factors (10, 20, 101, 202, 404, 505, 1010)

Kunal Das
Nov 2, 2013

a must be divisible by ( 2027 7 ) = 2020 (2027-7)=2020

2020 = 2 2 × 5 × 101 2020 = 2^{2} \times 5 \times 101

Distinct prime factors are 2 , 5 , 101 2, 5, 101

In a factor, count of number 2 can be 0, 1 or 2 - there are 3 ways.

Count of number 5 and 101 can be 0 or 1 - there are 2 ways for each.

So total number of factors are 2 × 2 × 3 = 12 2 \times 2 \times 3 = 12

This includes 1 (when all factors are present 0 times)

Of them, 1 , 2 , 5 , 2 × 2 = 4 1, 2, 5, 2 \times 2 = 4 these 4 factors which are less than 7.

Remaining 8 factors match our condition

Daniel Ferreira
Oct 31, 2013

Temos que: D = d q + r D = d \cdot q + r , então:

2027 = a q + 7 a q = 2020 a q = 2 2 5 101 2027 = a \cdot q + 7 \\\\ a \cdot q = 2020 \\\\ a \cdot q = 2^2 \cdot 5 \cdot 101

A quantidade de divisores é dada por:

( 2 + 1 ) ( 1 + 1 ) ( 1 + 1 ) = 3 2 2 = 12 (2 + 1) \cdot (1 + 1) \cdot (1 + 1) = \\\\ 3 \cdot 2 \cdot 2 = 12

Todavia, quatro deles são menores que 7, portanto,

12 4 = 8 12 - 4 = \\\\ \boxed{8}

Rutvik Paikine
Oct 30, 2013

Let quotient be x. Therefore 2027 is equal to [a.x] plus 7. ................as 7 is the remainder. So 2020 is equal to [a.x]. Therefore,factors of 2020 are : {1 . 2020}, {2 .1010} , {4 .505}, {5 .404}, {10 .202} and {20 .101} Therefore a can be 10,20,101,202,404,505,1010 and 2020 because dividing by 1, 2, 4 and 5 to 2027 won't bring remainder to 7!!!!

Therefore there are 8 values that satisfy the given condition.

Grant Goodman
Aug 18, 2013

Since 2020 is the largest possible value of a , all other values of a must give remainders of 0 when dividing 2020. Ergo, all values of a must be factors of 2020.

The factors of 2020 are 1, 2, 4, 5, 10, 20, 101, 202, 404, 505, 1010, and 2020. Note that only values larger than 7 can give 7 as the remainder, so the factors less than 7 are excluded.

This leaves 8 values: 10, 20, 101, 202, 404, 505, 1010, and 2020.

2027=ak+7 and a>7

ak=2020

ak=2*2*5*101

a can be

  • 101, (ignoring 2,2,5 because each of them are <7)
  • 2*5,2*101,5*101,
  • 2*2*5,2*2*101,2*5*101,
  • 2*2*5*101

so count =1+3+3+1=8

:)

I think you forget 2*2 <7 . :)

Kun Thong - 7 years, 10 months ago

You just have to see how many positive divisors of 2020 (which is 2027 - 7) greater than 7 there are. Since 2020 = 2^2 * 5 * 101, there are (2+1) (1+1) (1+1)=12 divisors of 2020. Since 1, 2, 4 and 5 are smaller than 7, there are 8 positive integrers a which leave a remainder of 7.

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