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A cuboidal aquarium, of base dimensions $100\;\text{cm} \times 80\;\text{cm}$ and height $60\;\text{cm}$, is filled with water to its brim. The aquarium is now tilted along one of the $80\;\text{cm}$ edges and the water begin to spill. The tilting is continued till the water surface touches a line on the base which is at a distance of one-third of the length from (and parallel to) the edge on which the aquarium is being tilted. Now the box is returned to its original position. By how many centimeters has the height of water reduced?

1. $50$
2. $40$
3. $20$
4. $10$

1