Let h be the height of the water in the tube. Then the total volume of water is
(h m) (1 m²) = h m³
After dropping the ball into the water, the height of the water changes to
h m + 2.25 mm = (h + 0.0025) m
and so the contents of the tub take up (h + 0.0025) m³ of volume.
This means the ball displaces
(h + 0.0025) m³ - h m³ = 0.0025 m³
of water, which is to say that this is volume of the ball.
With a mass of 4 kg, you would find its density to be
(4 kg) / (0.0025 m³) = (4 kg) / (2.5 L) = 1.6 kg/L
