Question

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<100, then k+n=?
17
16
15
14
13

Answer 1

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<n<100）于是n=14 故答案为17

希望满意~~~