Too much sand.
Volume is the amount of space something takes up. The formula for volume is v=length x width x height.
The volume of the sandbox is 36 ft^3, because 5x6x1.2 = 36. This means the box takes up 36 cubic feet of space.
If the customer bought 40 cubic feet of sand, than they bought to much because 40 > 36. The amount of sand she got is greater than the volume of the box, so it won’t all fit.
