# Math Is Fun Forum

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## #1 2006-11-20 05:27:25

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### whats the remain

a number divided by 7 and the remain is = 3 ... and another number divided by 7 also and the remain is = 5 ... whats the remain when we divide the product of these two numbers by 7 ?

Last edited by RauLiTo (2006-11-20 05:33:38)

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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## #2 2006-11-20 06:45:40

pi man
Member
Registered: 2006-07-06
Posts: 251

### Re: whats the remain

Let X be the first number with a remainder of 3.    So X = 7a + 3
Let Y be the second number with a remainder of 5.   So Y = 7b + 5

XY = (7a+3) * (7b+5)
= 49ab + 35a + 21b + 15

49ab, 35a, and 21b are all divisible by 7.   We only need to worry about the 15, which is 2*7 with a remainder of 1.

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## #3 2006-11-20 08:08:19

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### Re: whats the remain

lol thats easy ! thanks pi

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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