Verified answer

Area of the base of the tank is 100cm x 40cm = 4000cm^2

200 litres = 200,000 cm ^3

so the water level would be 200,000 / 4000 = 50cm

and 52cm after the stones

So basically, first step is to convert everything in the same unit (meters make it easier):

height= 0,55 m

width= 1m

depth=0,4 m

You have the volume of the water: 200L=0.2 m3

Knowing that the volume formula is : height x width x depth and you need to calculate the depth of the water you get: 0,55 x 1 x d =0,2 ; d= 0,2/0,55=0.36 (ish) m

So, the water height before you add the stones is 36 cm. Add the stones, 38 cm.