选择P输出的n个项目，这是公式来告诉你有多少组合也有。

Choosing p out of n items, this is the formula to tell you how many combinations there are.

                  n!
n choose p  = -----------
               p! (n-p)!

谷歌计算器会做数学题给你：

Google calculator will do the math for you:

http://www.google.com/search?q=6+选择+ 4

6选4 = 15