How many whole numbers between $$5000$$ and $$6000$$ consist of four different digits that decrease from left to right? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
The only such numbers are $$5432$$, $$5431$$, $$5430$$, $$5421$$, $$5420$$, $$5410$$, $$5321$$, $$5320$$, $$5310$$, and $$5210$$. In all, there are $$10$$ such numbers. 