help math question

来源: 于 2014-03-11 17:56:39 [旧帖] [给我悄悄话] 本文已被阅读：次
A number is called a decreasing number if it has two or more digits and each digit is less than the digit to its left. For example, 7,421; 964,310; and 52 are decreasing numbers but 3,421; 6,642; 8; and 963,212 are not. How many  5 digits decreasing numbers are there?

Can anyone explain in a way 10 years old can understand?

Thanks