15c
The team has fifteen members and each member sells an unknown amount of coupon books.
A + B + C = 518 ft^3
A = 1/3B, or 3A = B
B = C
A + B + C = 518 ft^3
1/3B + B + B = 518
7/3B = 518
B = 518×3/7 = 222 ft^3
A = 1/3B = 1/3×222 = 74 ft^3
C = B = 222 ft^3
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