When the tank is tilted, the water level reaches the midpoint of the 60 cm side, which is 30 cm from the bottom of the tank. This means that the height of the water is 30 cm.
To find the depth of the water when the tank is returned to its horizontal position, we need to use the concept of similar triangles. The two triangles formed by the water level and the sides of the tank are similar, because they have the same angles.
Let x be the depth of the water in centimeters when the tank is returned to its horizontal position. Then we have:
(40 - x) / 60 = 30 / 50
Simplifying this equation, we get:
40 - x = 36
x = 4
Therefore, the depth of the water in the fish tank when it is returned to its horizontal position is 4 cm.