A log of wood of mass 120 Kg floats in water. The weight that can be put on the raft to make it just sink, should be (density of wood=600Kg/m3)
Volume of log of wood V=massdensity=120600=0.2m3
Let x weight that can be put on the log of wood.
So weight of the body = (120+x)×10 N
Weight of displaced liquid = Vρg=0.2×103×10 N
The body will just sink in liquid if the weight of the body will be equal to the weight of displaced liquid.
(120+x) × 10N=0.2×103×10 N ⇒ x=80kg