TT GWD-2-13,讨论过，但解释的不太明白？

An integer greater than 1 that is not prime is called composite. if the two-digit integer n is greater than 20, is n composite?

1. the tens digit of n is a factor of the units digit of n.

2. the tens digit of n is 2

a. statement 1 alone is sufficient, but statement 2 alone is not sufficient.

b. statement 2 alone is sufficient, but statement 1 alone is not sufficient.

c. both statements together are sufficient, but neither statement alone is sufficient.

d. each statement alone is sufficient.

e. statements 1 and 2 together are not sufficient.

答案是A，有过这种解法 N= 10*a+a*m=(10+m)*a， 然后就下结论，此数一定为 合数，为什么？

先明确什么样的数是合数：就是除了1和这个数本身，如果它还可以被其它数整除，那么此数就是合数。

这道题目条件一已知：十位数（a）是个位数的因子，即个位的数可以整除十位的数，因此个位的数一定能表示成m*a 所以N可以表示成 N= 10a+ m*a =a*（10+m）

也就是说N可以被a、（10+m）、1、N整除。对照划线部分合数的定义得知，N是合数。

thank u. very clear and easy-understand.

不客气,互相帮助!

An integer greater than 1 that is not prime is called composite.  If the two-digit integer n is greater than 20, is n composite?

(1)     The tens digit of n is a factor of the units digit of n.

(2)     The tens digit of n is 2.

大家看看我哪儿理解错了：

举例 22和24，满足题目要求：n is greater than 20，并且tens digit of n is a factor of the units digit of n.

                        22+1=23 那么22就不是com

                        24+1=25 那么24就是com 

1条件不充分啊

我选的E

An integer greater than 1 that is not prime is called composite.  If the two-digit integer n is greater than 20, is n composite?

(1)     The tens digit of n is a factor of the units digit of n.

(2)     The tens digit of n is 2.

大家看看我哪儿理解错了：

举例 22和24，满足题目要求：n is greater than 20，并且tens digit of n is a factor of the units digit of n.

                        22+1=23 那么22就不是com

                        24+1=25 那么24就是com 

1条件不充分啊

我选的E

小狗，你好像理解错了。An integer greater than 1 that is not prime is called composite的意思是说，任何一个比1大而又不是质数的东东偶们叫它做合数。你举的22跟24都是合数啊，有了条件1，那么个位跟十位数合起来至少有一个factor是大于1而又不是本身的，所以A就充分啦！

希望我说明白了

ft

thx jojo