Solution to the Fish Tank Problem.

As the 60 cm edge is always horizontal, we can forget about it and reduce the problem to a two-dimensional question. When the tank is tilted the water shape against the front wall forms a triangle. When the tank is at rest upon its base the water shape is a rectangle. To keep constant the water volume the triangle and the rectangle must have the same area, thus:

area rectangle = area triangle

from the problem data:

100·water height = (40·50)/2

Hence:

water height = (40·50)/(2·100) = 10

all units are expressed in cm so the answer will be 10 cm.

Congratulations Jonathan!!!