GWD 1 - 31 & 37

Q31:

A positive integer n is said to be “prime-saturated” if the product of all the different positive prime factors of n is less than the square root of n.  What is the greatest two-digit prime-saturated integer?

A.     99

B.      98

C.      97

D.     96

E.      95

这题正确答案是D, 我不知道， 该怎么算。我把每个选项都分解了分解到质数。可每个选项都不能被完全平方根呀。所以就不知道怎么解了。还是说我没读懂题。

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

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 这题选B, 我是半蒙地，但是我怎么算都是1/10, 不是1/20.

4*3 / [c(16, 2)]=1/10.

请高人帮忙，指教，谢谢。

31题，只要the product of all the different positive prime factors of n is less than the square root of n就OK了，不用管n本身能不能开平方。

37题，你计算分母的时候不必考虑排列组合的问题，只需要先从16个里面选一个，再从剩下的15个里面选一个，所以得到16*15，分子是4*3，毫无疑问，就这样。

楼主几号考？加油！

Q31

99    3*11>V99

98    2*7>v98

97    97>v97

96    2*3<v96

Q37

first step:4/16

second step: 3/15

4/16*3/15=1/20

多谢，多谢。

我21日，虽然复习的不怎么样，哎，还是想搏一次了