What is Centre of gravity of rectangular box?

Answer 1

It's the point inside the box that's halfway between the left side and right side,

halfway between the front and back, and halfway between the top and bottom.

Stated another way, the Center of Gravity is at the Center of Mass.

If we replace the box with a perfectly spherical bubble, the Center of Mass is at the very middle of the bubble.

Physics doesn't care if the center is empty, solid or creme filled. The very middle of that bubble is still the Center of Mass and therefore the Center of Gravity