Answer:
224 boxes
Imagine a cuboid with the dimensions 7 feet by 4 feet by 8 feet.
To find volume of cuboid: l x w x h (length, width, height)
7 x 4 x 8 = 224^3 (cubed)
Considering each box is 1 foot cubed, this means you can fit 224 boxes within the back of the truck.
Hope that makes a bit of sense ;)