其它
In how many ways can the letters in $AAABCDA$ be rearranged so that two or more $A$s do not appear together? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations
There are $4$ $A$s in total, which have $3$ intervals leaving for the other $3$ letters. Thus, the answer is $\_3P\_3=6$. 