4spoons and 3 forks cost $15 4 spoons and 1 fork cost $13 how many do the forks cost?

Let's say that s=spoons and f=forks. 

According to this, we can say that:

{4s+3f=15
{4s+f=13

This is a system. 

We can then use elimination to solve and isolate the variable:
(4s-3f=15)
- (4s+f=13)

We can then subtract and conclude that:
-4f=2
-f=1/2
f=-1/2