求助 gwd-7-14

renprince

GWD7

数学第14题, 请问哪位能告诉我是怎么算出来的?谢谢

Q14:

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 ?

答案: 2/5

sammen

一共的取法有,2-6;3-5;4-4;5-3;6-2有5的那就是2/5啊.只要注意4-4只有1次就可以了.

creativity

Here are the 5 pairs: (2,6), (3,5), (4,4), (5,3), (6,2) with the same probability to be drawn. Since "5" is in 2 pairs, the answer is 2/5.