Answer: 4cm
Explanation:
Ok so no matter the orientation of the carton, it will contain the same volume of milk. We can use the fact it's volume of milk will stay constant to find out it's new depth.
Before being turned over the milk volume is:
5 * 8 * 12 = 480. This is because the volume of a cuboid is length * width * height (depth).
Therefore the volume of the milk once turned over is 480
When on it's side, the volume of the milk equals
8 * 15 (the base) * depth
120 * depth
120 * depth = 480
so the depth = 4cm