A raft of wood is 2.5 m × 1.6 m and 0.25 m thick.

Step 1. (a) Volume $= 2.5\ \text{m} \times 1.6\ \text{m} \times 0.25\ \text{m} = 1.0\ \text{m}^3$, so the mass is $600\ \text{kg}$.

Step 2. (b) $0.6 \times 0.25\ \text{m} = 0.15\ \text{m}$ under water.

Step 3. (c) Fully under, the water holds up $1000\ \text{kg}$; the raft is $600$ of that, so $400\ \text{kg}$ more.