Question 1127300
O = number of orchestra seats sold
B = number of balcony seats sold<br>

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O + B = 120         (1)  (from the number of tickets sold info)
18O + 12B = 1920    (2)  (from the revenue info)<br>
</pre>
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)