If $$20$$ years ago Allen was half as old as he is today, then today he is $$40$$. Thus, $$10$$ years ago he was $$30$$. 
If $$20$$ years ago Allen was half as old as he is today, how old was he $$10$$ years ago? 

1) $$20$$ 
2) $$30$$ 
3) $$40$$ 
4) $$50$$ 