问题如下：
There are 15 students in a race of the sports. the winners will be awarded the $100, 50$, and $10. How many different ways do they have?
选项：
A. 45.
B. 2730
C. 455.
解释：
B is correct.
In this case, the order is important, we use the permutation formula to solve this problem.
3是怎么来的。请帮忙解释下。谢谢。
