A storage tank is in the form of a cube.

Let side of a cube be = x m

∴ Volume of cubical tank = 15.625 m3 [given]

=> x3 = 15.625 m3

=> x = 2.5 m

and present depth of water in cubical tank = 1.3 m

∴ Height of water used =2.5 – 1.3 m = 1.2 m

Now, volume of water used = 1.2 x 2.5 x 2.5= 7.5 m3

= 7. 5 x 1000 = 7500 L [∴ 1 m3 = 1000 L]

Hence, the volume of water already used from the tank is 7500 L.