其它
How many different four-digit numbers can be formed by rearranging the four digits in $2021$?~(Adapted from $2004$ AMC $8$ Problem, Question \#2) 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations
When the thousand digit is $2$, there are $\_3P\_3=6$ ways. When the thousand digit is $1$, there are $\_3C\_1=3$ ways. So the answer is $3+6=9$. 