O = number of orchestra seats sold
B = number of balcony seats sold
O + B = 120 (1) (from the number of tickets sold info)
18O + 12B = 1920 (2) (from the revenue info)
Will solve by substitution. (1) implies O = 120 - B. Substitute "120-B" for "O" in (2):
18(120-B) + 12B = 1920
2160 - 18B + 12B = 1920
-6B = 1920 - 2160
-6B = -240 —> B = 240/6 = 40
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Ans: 40 balcony seats were sold
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Check: 40 balcony seats —> 80 orchestra seats (using (eq 1))
40*12 + 80*18 = 480 + 1440 = 1920 (revenue info checks out)