In two years my Cousin Marty will be twice as old as he was
five years ago. How old is Marty?

In 2008 Marty will be twice as old as he was in 2001.
How old is he in 2006?  

In 2006 Marty is M years old.
In 2008 Marty will be M+2 years old
In 2001 Marty was M-5 years old

Therefore M+2 must be twice M-5

M+2 = 2(M-5)

Solve that and you'll get that Marty is 12 in 2006.  
Checking: In 2001 Marty was 7 and in 2008 he'll be 14, 
and that's twice 7.

Edwin
AnlytcPhil@aol.com