Tilted tank

Can you find the height of the water in this tilted tank when it is flat?
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A fish tank is a cuboid, 100 cm by 60 cm by 40 cm. It is partly filled with water, so that when it is resting on a 60 cm edge, the water level reaches the top edge and the midpoint of the base, as shown.

Image
Tilted tank
 

What will the height of the water be when the tank is resting on its base (a 100 cm by 60 cm face)?

This problem is adapted from the World Mathematics Championships