You have a rectangular aquarium, whose bottom face is a 60 cm × 60 cm square. You put another iron box, which has height 12 cm and a square bottom face of side length 15 cm, into the aquarium. Now, you fill the aquarium 20 cm high with water. After you take out the iron box, what is the depth of the water?

cm

The aquarium has a volume of 60 × 60 × 20 = 72000 cm^{3}
of water. However, we need to subtract out the iron that is in the water but
not water. This iron has a volume of 15 × 15 × 12 = 2700 cm^{3}. Thus,
the actual volume of water is 72000 - 2700 = 69300 cm^{3}. Thus, after
the iron box is gone, the depth of the water × 60 × 60 = 67500, so the depth of
the water is 69300 ÷ 3600 = 19.25 cm.