急求本月jj中一类题目的思路，很快考了，谢谢大家

kcheong15

以下是引用zhangxiaofff在2006-12-15 17:40:00的发言：

108. 一个数除以10的余数是多少？1) 除以5余2  2) 除以2余1，答案：C
答案：C（余7）

这类题目是啥思路来，我每次一看就晕了，急求大虾相助，非常感谢

(1) a number divided by 5 has reminder = 2, means the number must have unit digit = 2 or 7 (e.g. 7, 12, 17, 22....)

It is not sufficient to answer the question, as 12/10, reminder = 2.  17/10, reminder = 7.

(2) the number divided by 2 will have reminder = 1.  This means the number is odd number.

(2) alone cannot answer the question.

But combining (1) and (2)...,

So we can tell the number must have unit digit = 7.  When 17 is divided by 10, the reminder must be 7.

So answer is (c).