请教两道数学题

cageshi117

1.A gardener is going to plant 2 red rosebushes and 2 white rosebushes.  If the gardener is to select each of the bushes at random, one at a time, and plant them in a row, what is the probability that the 2 rosebushes in the middle of the row will be the red rosebushes?答案是B

 

A.      1/12

B.      1/6

C.      1/5

D.      1/3

E.      1/2

2.If n is the product of the integers from1 to 20 inclusive, what is the greatest integer k for which 2^k is a factor of n?答案是D

 

A.      10

B.      12

C.      15

D.      18

E.      20

MRFROG

第一题就是说gardener抽到按照white,red,red,white顺序来排列的概率是多少？(2/4)*(2/3)*(1/2)*(1/1)=1/6

第二题要看n中有多少能被2整除，n中的奇数肯定不可以，那们只有2。。20十个偶数，这10个偶数可以写成2，2^2，2*3，2^3，2*5，2*2*3，2*7，2^4, 2*9,2*2*5,这些2的因子加起来刚好有18个。

cageshi117

感谢~