Answer:
First, you find the volume of each box
Volume = Length * Width * Height
Box A
V = 13 * 40 * 5
V = 2600 cm^3
Box B
V = 8 * 10 * 25
V = 2000 cm ^3
Box C
V = 11 * 20 * 30
V = 6600 cm ^3
Since 7400, the cubes trying to be packed, is bigger than any of the volumes of the boxes, 2 boxes must be used
The boxes with volumes that add up to a number greater than 7400 are Box C with Box A or B (either works, as long as C is one of them)
The total volume of the boxes together can be found with simple addition
2600 + 2000 + 6600 = 11200 cm ^3