Question 131074
lets say hamburgers sold is H and cheeseburgers sold is C.

Then we are given:
H+C=639
H-61=C

We need to find H.

One way to solve is to plug in C from the bottom equation into the top one:
H+H-61=639

So now we can solve for H:
2H-61=639
2H=639+61
H=(639+61)/2