Density = Mass/Volume
For the sealed tin
Mass= 250 g
Volume= 400 cm³
Now, we know that density of water is 1000 kg/cm³ or 1 g/cm³
As the density of the sealed tin is less than the density of water, it will float.
It can be done by method as follows:
Specific Gravity (Relative Density with respect to water) = Density of sealed tin/Density of water
Thus the specific gravity of the sealed tin is 0.625
So it floats on water.