A 3 kg block of volume 0.004 m³ is held fully under water…

Step 1. (a) $1000\ \text{kg/m}^3 \times 0.004\ \text{m}^3 \times 9.8\ \text{m/s}^2 = 39.2\ \text{N}$.

Step 2. (b) $3\ \text{kg} \times 9.8\ \text{m/s}^2 = 29.4\ \text{N}$.

Step 3. (c) The upward $39.2\ \text{N}$ beats the $29.4\ \text{N}$ weight, so it rises and floats. Its density is $750\ \text{kg/m}^3$, so $75\%$ of it stays under the water.