题目内容：

求助几个GRE的数学题
1,from the set of 6 letters,A,B,C,D,E and F,there are 20 different 3-letter subsets that could be selected.
A:the number of 3-letter subsets that include F
B:10
2,10,15,25,30,x
if d is the standard deviation of the numbers in the list above,for which of the following values of x would the value of d be least?
0
5
20
30
80
3,
A:the least positive integer with four different prime factors,each greater than 2
B:1,155
4,which of the following can't be a factor of 2n,3k(2的n次方乘以3的k次方),where n and k are positive integers?
6
8
27
42
54
5,if 72.42=k(24+n/100),where k and n are positive integers and n

优质解答

1 六个字母里选出三个字母,其中包括F的有几组.问题等价于从剩余五个字母中选两个 答案为 c(5,2) = 10组
2 标准差要小,所以x要尽量接近前面数字的平均值,平均值为20 故选20
3 求有四个质因数的最小整数：3*5*7*11=1155
4 如果一个数可以成为2^n3^k的因数,那么它必须能表示成2^x3^y的形式(x y 可以为0)因此42不行 (6 = 2^1 3^1,8 = 2^3 3^0,27 = 2^0 3^3,54=2^1 3^3)
5 要满足等式 k=3（其他值都不能满足0