请教gwd-28-6

camilla2006

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.

                  

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.

                                                                                                  Answer: d

答案怎么是d呢？

1）--〉C(N,2)/C(2,10)=1/15-->N=2 OR 3    不充分

2）--> C(1,10-N)C(1,N)/C(2,10)=7/15-->N=3 OR 7, because "fewer than half of which are defective", so N=3, 充分。

请给看看我的推理正确吗？

kdjd

N=2不是 1）的解，解唯一即 N=3