Length & breadth of the aquarium = 40cm(as it is square)
Volume of aquarium = l x b x h
volume of aquarium = 40 x 40 x 50 = 80000 cm³
Volume of water = ½ half of aquarium
Volume of water = ½ x 80000 = 40000 cm³
Volume of the solid cube = a³ = (20cm)³ = 8000cm³
Surely, 40000>>>8000, level doesnt reach the top of the tank.
Volume ocquired = 40000+8000=48000 cm³
Now, let it has reached reached a height h from the ground(bottom).
Volume = 48000 => lx b x h = 48000
=> 40 x 40 x h = 48000
=> h = 30 cm
The height at which water has reached is 30 cm(from bottom).
Thus, below the top it has reached = total height - 30 cm = 50 - 30 = 20 cm
