余数问题

每次遇到都是很清楚如何解.

When positive integer x is divided by 5, the remainder is 3; and when x is divided by 7, the remainder is 4.  When positive integer y is divided by 5, the remainder is 3; and when y is divided by 7, the remainder is 4.  If x > y, which of the following must be a factor of x - y?

A.     12

B.     15

C.     20

D.     28

E.      35

答案: E

这种题有没有什么固定的思路呀...还是就是乱试...

两个数被5除，余数相同，那么这两个数相减，结果肯定是5的倍数；

同理，也是7的倍数

结合起来就可以确定答案e