Let b = baseball, k = basketball, t = tennis ball, s = soccer ball.
Then you have the following four equations:
b + k = 240 (1)
t + s = 500 (2)
s - k + t = 455 (3)
b + t = 215 (4)
From (1) we have k = 240 - b. Put this into (3):
s - 240 + b + t = 455 Rewrite as:
s + t + b = 695 (5)
Use (2) in (5) to get 500 + b = 695 so that b = 195
Use b in (4) to get 195 + t = 215 so t = 20
Use t in (2) to get 20 + s = 500 so s = 480
The socer ball is worth 480.
