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标题: 求教 TT GWD-12-6 [打印本页]

作者: lguo1 时间: 2006-10-20 13:38
标题: 求教 TT GWD-12-6

Q6:

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.

The given answer is D. However I cannot use condition 2 to get any integer value solution.

From condition 2, I got (n/10) x (10-n)/9 = 7/15 ====> n x n - 10n + 42 = 0 ===> no integer solution

Did I get anything wrong?

Thanks!

作者: bluegold998 时间: 2006-10-28 14:06
你的方程错了，应该是[(n) x (10-n)]*2/(9*10)=7/15。你少乘了一个2。

作者: qiuhailing01 时间: 2009-4-12 21:37

以下是引用bluegold998在2006-10-28 14:06:00的发言：
你的方程错了，应该是[(n) x (10-n)]*2/(9*10)=7/15。你少乘了一个2。

这不是有两个解么： 3和7，不符合唯一性原则啊。。。。

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