price of paperback = p
price of hardcover = h
Q: p + h = ?

(1) 2p + 3h = 12.50
(2) 4p + 6h = 25.00

If you divide (2) throughout by 2 then you get (1).
this simply means there is only one equation to solve 2 variables - which is simply not enough. hence the answer is (E)