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Just tried googling but couldn't find any example, but how 3 % 5 = 3

Googled it

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If you count $1,2,3,4,5,1,2,3,4,5,1,2,3,4,5,1,2,3,\ldots$, what is the third number that comes up? –  Michael Greinecker Sep 12 '12 at 10:30
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When you divide $3$ by $5$, the remainder is $3$. –  Joe Johnson 126 Sep 12 '12 at 10:31
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Also, $3 \equiv 3 \mod 5$, because $3-3=0 \equiv 0 \mod 5$. –  Andrea Orta Sep 12 '12 at 10:34
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If you are asking this question $\implies$ you need to look at the definition of modulo operation. see en.wikipedia.org/wiki/Modulo_operation –  Aang Sep 12 '12 at 12:45

5 Answers 5

up vote 7 down vote accepted

If $a$ and $b$ are positive integers, there exist unique integers $q$, $r$ with $$a = bq + r$$ and $0 \leq r < b$.

This theorem is called the division algorithm, and $a\ \%\ b$ is defined to be this $r$. In your case, $3 = 0\cdot5 + 3$ and $0 \leq 3 < 5$, so the answer is 3.

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hmmm Actually Mod (%) returns the remainder Given two positive numbers, a (the dividend) and b (the divisor), a modulus % is the remainder of the Euclidean division of a by b. For instance, the expression "9 mod 8" would evaluate to 1 because 9divided by 8 leaves a remainder of 1, while "9 mod 3" would evaluate to 0 because the division of 9 by 3 leaves a remainder of 0. hope this will help you cheers

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plus if your dividend if less than the divisor, then your remainder will be the dividend :) –  Ali Sep 12 '12 at 10:52
    
It works for negative numbers too, unless you are using a non-standard definition on Mod? –  user1729 Sep 12 '12 at 13:02

$a \equiv b \, (\text{mod} c)$ means $a-b$ is divisible by $c$. [Definition]

Since $3 - 3 = 0$ is divisible by $5$, we have $3 \equiv 3 \, (\text{mod} 5)$.

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a modulo b = r Where r is the remainder of the division a by b => a = b*q + r where q is the quotient of the division a/b.

For example: 9 modulo 4 = 1 and the quotient is 2

i.e 9 = (4*2) + 1

In a similar fashion: 3 modulo 5 = 3 and the quotient is 0- =>3=(5*0)+3

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by definition, the remainder of a division is the fractional part of a division. if you have 3/5 (3 divided by 5), you have the fractional part represented as 3/5. since a remainder is the dividend or numerator of a fraction, you just take 3 as your remainder (the divisor or denominator is not mentioned in the remainder).
therefore, 3 % 5 = 3.

just like 5 % 3 = dividing 5 by 3, you get 1 and 2/5. to get the remainder part, you know that 2 is the numerator so 2 is the remainder (the denominator, 5, is not mentioned in the remainder).

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