How many three-digit whole numbers have a ones digit equal to the sum of the hundreds digit and the tens digit? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
If the ones digit is $$2$$, the $$2$$ numbers are $$202$$ and $$112$$. For each ones digit, the number of possible numbers is the same as the ones digit. In all, there are $$1+2+3+\cdots+8+9 =45$$ numbers. 