Calculate volume of wood.
Given,
Mass of wood =120 kg
Let the volume of wood be V.
Therefore,
Weight =pwoodVg
120g=600Vg
V=15m3
Calculate mass required.
Let the mass be added is m, therefore
Total weight = buoyancy
pwoodVg+mg=pwVg
600Vg+mg=1000Vg
m=400×15
m=80 kg