问一道题：NO.PZ2017092702000077

问题如下：

A firm will select two of four vice presidents to be added to the investment committee. How many different groups of two are possible?

选项：

6

12

24

解释：

A is correct.

The answer is found using the combination formula ${}_nC_r={(\begin{array}{c}n\\r\end{array})}=\frac{n!}{{(n-r)}!r!}$ Here, n = 4 and r = 2, so the answer is 4!/[(4 – 2)!2!] = 24/(2) × (2) = 6. This result can be verified by assuming there are four vice presidents, VP1–VP4. The six possible additions to the investment committee are VP1 and VP2, VP1 and VP3, VP1 and VP4, VP2 and VP3, VP2 and VP4, and VP3 and VP4.