A storage tank is in the form of a cube. When it is full of water, the volume of water is 15.625 m3. If the present depth of water is 1.3 m, then find the volume of water already used from the tank.
When the cubical tank is full of water:
Volume of water = Volume of cube = 15.625 m3
Then, Volume of cube = (length of edge of cube)3
⇒ (length of edge of cube)3 = 16.625
⇒ length of edge of cube = 
= 2.5 m
Now we know the length of the edge of the cube, i.e., 2.5 m.
When the present depth of the water is 1.3 m:
Depth = 1.3 m
Length of tank = 2.5 m
Breadth of tank = 2.5 m
So, volume of water upto depth 1.3 m = length × breadth × depth
= 2.5 × 2.5 × 1.3
= 8.125 m3
Then, volume of water already used from the tank = (Volume of tank when it was full of water) – (Volume of water when depth is 1.3 m)
= 15.625 – 8.125
= 7.5 m3
Thus, volume of water already used from the tank is 7.5 m3.