This number is huge!

A = 12345678910111213 20122013201420152016 A = 12345678910111213 \ldots 20122013201420152016

Let A A be the concatenation of first 2016 positive numbers.

Find the remainder when A A is divided by 72.


The answer is 0.

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3 solutions

Peter Macgregor
Apr 1, 2016

Notice that 72 = 8 × 9 72=8\times9

We will show that the huge number H is divisible by both 8 and 9 and so it must be divisible by 72, and the remainder is zero.

First write H = k × 1000 + 16 = 8 × ( 125 k + 2 ) H=k\times1000+16=8\times(125k+2) to see thet H is divisible by 8.

The remainder when a number is divided by 9 equals its digital root. The digital root is the sum of all the digits of the number reduced modulo 9. The key insight is that the place values of the digits are irrelevant and so the digital root of H is equal to the digital root of

i = 1 2016 i = 1008 × 2017 \sum_{i=1}^{2016}i=1008\times2017

Since 1008 is a multiple of 9, the digital root of H is zero, or in other words H is divisible by 9.

Now since H is a multiple of both 8 and 9, and they are relatively prime, H is divisible by 72 and the required remainder is 0 \boxed{0}

Why? Are you mean the remainder of 201420152016/9 is (2014+2015+2016)/9 cause you used summation. But I think It's should (2+0+1+...+0+1+6)/9, isn't it?

joke jeab - 5 years, 2 months ago

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Yes you are right. I'm also confused about divisibility by 9.

Shubh Gulati - 5 years, 2 months ago

What if the remainder is not zero ? I mean like 1234...21062017 divided by 72 which apparently the remainder is not 0, how to do so.

冠曄 林 - 5 years, 2 months ago
Adarsh Kumar
Mar 31, 2016

The number is definitely divisible by 8 as the last three digits are. Now,the sum of the single digit numbers is divisible by 9. Now,the two digit numbers are all of the form a b = 10 a + b \overline{ab}=10a+b ,so for all values of a a ,the sum of the values of b b ,are divisible by 9 as it varies from 0-9.Similarly,for a a ,it also varies from 1-9,hence 10(1+2+..9) which is also divisible by 9.This can be done for 3 digit numbers too.I am currently on phone so will post the full solution later on.

Andrea Virgillito
Mar 21, 2017

72=2^3*3^2 The number above is divisible by 2^3 because the number formed by the last three digits is divisible by 8 and also by 9 because the sum of every set of 9 positive consecutive integers is divisible by 9 and there are 224 sets of nine positive consecutive integers :). Thus 12345678910111213...20122013201420152016 is divisible by 72.

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