(x)1100+(y)113 x depends on the amount of computers is sold so x will change. y will be the amount of hard drives sold and then you add the total amount together.
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