2 道数学麻烦帮忙~！

shain

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 ?


A.
1/6B.
1/5C.
1/3D.
2/5E.
2/3

Q36:
If a and b are positive integers such that a – b and
are both even integers, which of the following must be an odd integer?

a. a/2; b. b/2; c. (a+b)/2; d. (a+2)/2; e. (b+2)/2

shain

哦 明白了 非常感谢！！！我觉得我还是没看懂题~以为是抽到两个数等于八的时候其中一个数为5的情况占所有情况的几分之几。

alexandrajuan

2&6  算2种情况（顺序不同）
5&3   算2种情况（这里出现5的算两种情况）
4&4   算1种情况（顺序调换也一样）
共5种情况

所以 概率为 2/5

shain

第二题明白了~非常感谢！第一题能麻烦再解释一下么？谢谢！

shain

非常感谢！但第一题为什么选D呢？谢谢！

哦第二题是
Q36:
If a and b are positive integers such that a – b and a/b
are both even integers, which of the following must be an odd integer?

a. a/2; b. b/2; c. (a+b)/2; d. (a+2)/2; e. (b+2)/2

alexandrajuan

第一题选D
第二题不全吧？

选D
a-b=even
=> either both are even or both are odd

a/b=even=2x
=> a and b are both even
'a' must have atleast two 2s as factors.
so a/2 is even and a/2+1 is odd