求助涛涛-28-10

憨憨

Q10:

Six cards numbered from 1 to 6 are placed in an empty bowl.  First one card is drawn and then put back into the bowl; then a second card is drawn.  If the cards are drawn at random and if the sum of the numbers on the cards is 8, what is the probability that one of the two cards drawn is numbered 5 ?

A.     1/6  

B.     1/5  

C.1/3

D.     2/5 

E.2/3,  答案是D,不知是何思路，烦请那位高人给指点一二，下星期就考了，急， 万分感谢

observer

The probability is a ratio of two things. First, you need to compute the number of all possible ways. In this problem, how many ways are there to get a sum of "8." There are 5 ways: 2+6, 3+5, 4+4, 5+3, and 6+2.

Second, you need to know how many ways to achieve the goal -- to get a "5" in one of the two draws. Apparently, there are 2 different ways: 3+5 and 5+3.

So the answer is 2/5.