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H + C = 650 (1) ("a combined total of 650 hamburgers and cheeseburgers")
C = H - 50 (2) ("There were 50 fewer cheeseburgers sold than hamburgers.")
Substitute C = H - 50 into (2), based on (1).
You will get
H + (H-50) = 650 ====> 2H - 50 = 650 ====> 2H = 650+50 = 700 ====> H = 350 hamburgers.
Answer. 350 hamburgers and 300 cheeseburgers.