If he sells half of his stamps and buys 8 more after and ended up with 33 I would take away 8 (33-8=25) and get 25 (which is the other half that he didn't sell). After figuring out what the other half was you would double this number and get your starting number which is 50 ( 25*2= 50)
Note that one equation has -2y and the other has +2y. The fastest solution is to add the equations together. The two terms will cancel out, eliminating the y terms.
