Question

A storage tank is in the form of a cube. When it is full of water, the volume of water is 15.625m^3.
If the present depth of water is 1.3 m, find the volume of water already used from the tank.

Answer 1

volume of cubical tank = 15.625 m³
side³ =15.625
⇒side = 2.5 m
present depth = 1.3 m
so empty depth = 2.5 - 1.3 = 1.2 m
volume of water used = 2.5×2.5×1.2 = 7.5 m³ 

Answer 2

The volume of tank=15.625 m^3
So side of tank=(15.625)^1/3 m=2.5m
Now 
Depth of water =1.3m therefore
Depth of water used=2.5-1.3m=1.2m
So volume of water used=2.5×2.5×1.2m^3
=7.5 m^3
=7.5×1000 liter
=7500 litre