So a body floats when it displaces water equal to its own mass.
You know the density of water is 1 kg per litre (1000cubic centimetres) , I hope.
So the volume of 10kg of water is 10,000 ccs.
So this is the volume of water displaced and is the volume of the box below the water level.
So you need to solve the equation
h * 40 *20 = 10000
where h is the height the water comes up the side of the box in cm.
