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.
分不清贴标签和排列的区别
比如这道题： 从15个学生里面分类有不同3个奖金，其实不是和贴标签的原理一样吗
