使用排列和组合的智商问题

Question

使用数字54730的所有数字，可以创建小于70000且5的倍数的5个不同的5位数字？

Answer 1

您将加起来：

5xxx0 = 3! = 6 combinations that start with 5 and end with 0
4xxx0 = 3! = 6 combinations that start with 4 and end with 0
4xxx5 = 3! = 6 combinations that start with 4 and end with 5
3xxx0 = 3! = 6 combinations that start with 3 and end with 0
3xxx5 = 3! = 6 combinations that start with 3 and end with 5

6*5 = 30 total combinations

注意： 您不会执行7xxx0或7xxx5，因为它们会大于70000

来源： https://en.wikipedia.org/wiki/Combination#Number_of_k-combinations_for_all_k