Só complementando. Vejam o trecho que copie do livro do Kernighan and Ritchie:
"...The binary / operator yields the quotient, and the % operator the remainder, of the division of the first operand by the second; if the second operand is 0, the result is undefined. Otherwise, it is always true that (a/b)*b + a%b is equal to a . If both operands are non-negative, then the remainder is non-negative and smaller than the divisor, if not, it is guaranteed only that the absolute value of the remainder is smaller than the absolute value of the divisor...."