You can put this solution on YOUR website! Something is wrong with this problem.
There is eith not enough stamps or the $1.60 is too much to pay for 4 & 5 cent stamps.
Proof: if you by (30) 5 cent stamps you will spend only $1.50 which is the maximum amount.
You can't buy 30 4 or 5 cent stamps for $ 1.60.