Volume of a pyramid = 1/3 x Area of base x height
= 1/3 x 900 x 40
= 12000 cm^3
Volume of tank = length x breadth x height
= 40 x 40 x 40
= 64000 cm^3
64000 - 12000 = 52000 cm^3 water left
52000 cm^3 / 40 x 40 = h
h = 32.5cm
So, the depth of water in the tank is 32.5 cm. Hopefully that helped!