其它
How many different four-digit numbers can be formed by rearranging the four digits in $2004$?~ 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations
0 shouldn\textquotesingle t be put in the highest digit. There are two 0s  The solutions are: 2400, 2040, 2004, 4200, 4020, 4002. 