GWD1的一道概率题

cll0503

Q37:

A jar contains 16 marbles, of which 4 are red, 3 are blue, and the rest are yellow.  If 2 marbles are to be selected at random from the jar, one at a time without being replaced, what is the probability that the first marble selected will be red and the second marble selected will be blue?

 

A.      3/64

B.      1/20

C.      1/16

D.     1/12

E.      1/8

我是直接3/16乘以4/16算出来的，结果是A，但是正确答案是B，请问如何求出？

cnzalieen

第一次拿了一个就少了一个了所以第二次是15个的总数了4/16*3/15=1/20

cll0503

天，竟然是这样！