this was on a math test and I figured out the answer but not how to form a algebraic expression to solve it.**A farmer has chickens and pigs that total 28. There are a total of 72 Legs. ****How many pigs does he have?****Use an algebraic expression to find the answer.**

I could not for the life of me figure out any sort of expression for this and I took the test last thursday and it still bugs me. I blame my teacher throwing us a curve ball.