求助几个GRE的数学题

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