What is the volume of the box?

A normal-level "Volume" problem.In cut-and-fold problems the loss happens at both ends. Subtracting only one end is by far the commonest mistake.

GivenA square sheet of paper, side 20 cm. Squares of side 4 cm are cut from the four corners and the flaps folded up to make an open box.

Hints

Cut off the four red corners and fold the remaining flaps upwards. The middle square becomes the floor of the box.
This is where people slip. The cut happens at both ends, not just one. So the floor's side is 20 − 4 × 2 = 12 cm, not 20 − 4.
The floor area is 12 × 12 = 144 cm².
The height of the box is the width of the folded-up flap — that is, exactly the 4 cm that was cut away.
Now just "base area × height": 144 × 4 = 576 cm³.

Answer 576cm³