If $$20$$ years ago Allen was half as old as he is today, how old was he $$10$$ years ago? 
Overseas Competition->Knowledge Point->Word Problem Modules->Age Problems->When..., When... Type Age Problems
If $$20$$ years ago Allen was half as old as he is today, then today he is $$40$$. Thus, $$10$$ years ago he was $$30$$. 