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March 6, 2015

Posted by **yin** on Wednesday, November 28, 2012 at 10:46am.

your pupils in rows of five, two are left over. If you group them in eights, only one is left over. So you

try it with rows of nine, but now you are left with seven pupils. What do you think would happen if

you grouped your pupils in rows of eleven? And can you guess how many pupils you have?

- Proofs and numbers -
**Reiny**, Wednesday, November 28, 2012 at 11:56amYou could do it by brute force

conditon 1: remainder of 2 when divided by 5 ...

7 12 17 22 27 32 ... 82 87 92 97 102 .....

condition 2: remainder of 1 when divided by 8

9 17 25 33 41 .... 89 97 105 ...

condition 3: remainder of 7 when divided by 9

16 25 34 .... 88 97 106 ...

ahhh, looks like 97 is in all three cases

So there could be 97 students

If grouped in rows of 11 , the remainder would be 9

A more rigorous method is to use the

"Chinese Remainder Theorem"

It is a rather complicated method and you must know how to manipulate expressions such as

a mod(b)

e.g. 47 = 2mod9

- Proofs and numbers -
**Reiny**, Wednesday, November 28, 2012 at 12:11pmIf you want to know more about the Chinese Remainder Theorem , here is a good YouTube for it

http://www.youtube.com/watch?v=3PkxN_r9up8

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