Answer: 350
Step-by-step explanation: Let's say H is the number of hamburgers and C is the amount of cheeseburgers. H + C = 645. It says that there are 55 fewer cheeseburgers sold than hamburgers. So C = H - 55. If you substitute C for H - 55 your equation should look like
H - 55 + H = 645. Then you combine your H's, so the equation should be 2H - 55 = 645. Then you add 55 to both sides and you end up with 2H = 700. Divide by two and you get 350 for the amount of hamburgers sold.
To make sure you are correct you can plug in 350 for H and solve for C. So C = 295 and
295 + 350 = 645.