A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously from the box. If n of the bulbs in box are defective, what is the value of n?

(1) The probability that the two bulbs to be drawn will be defective is 1/15.

(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.

（2）的条件算出来，N是无解，这样也算充分吗？

我昨天也遇到这题,点了A,不对

(1): 根据条件可知有3个坏的

(2) 后来我又这样算,看对不对:

2个灯泡一个好的一个缺陷的,可以这样出现好,坏)或(坏,好)

假设有7个好的,3个坏的. 抽2个出现一好一坏的概率:2*(7/10)*(3/9)=7/15

所以(2)也是充分的.

答案为D

I get it ,thanks !!