First of all, you have to understand that the water level rises a lot when the big cube is put in, because the mass of the big cube is large. The height at which the water level rises is proportional to the mass of the cube. After being cut into small cubes, the mass is small. Assuming that the water surface is wide enough, every small cube can touch the water surface. In this way, the volume of boiling water discharged by each small cube is only proportional to the mass of this small cube, which is no longer 0.6 meters. That is to say, if all the small cubes are put in water, the height of boiling water is not equal to the height of water displaced by the whole big cube.
This conclusion can also be derived from the buoyancy formula. F=pVg, where p is density and g is gravity, and these two are constants. Now let's look at the volume v of boiling water. No matter whether it is a big cube or all the small cubes, the mass is equal and the buoyancy should be equal, so the volume of boiling water discharged by them should be equal. V=sh, s is the bottom area, and h is the height of boiled water. After the big cube is cut into small cubes, the contact area with water will definitely increase, so the height h will naturally become smaller. Do you understand?
2. Is the bottom of the small cube in direct contact with water?
What do you want to calculate if you don't know this question? But in this case, only a few quantities are involved: according to the balance of forces, the gravity of the object = the buoyancy of the object+the supporting force of the bottom to the object.
3. From the point of view of physics, it is not rigorous strictly. He wants to make sure that the water surface is big enough to put down every small cube and the water is deep enough.