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 = 600 Kg/m³)

Volume of log of wood  $V=\frac{\text{mass}}{\text{density}}=\frac{120}{600}$=0.2 m³
Let x weight that can be put on the log of wood. 
So weight of the body =  $(120+x)\times 10N$
Weight of displaced liquid  =   $V\sigma g=0.2\times {10}^3\times 10N$
The body will just sink in liquid if the weight of the body will be equal to the weight of displaced liquid.

$\begin{array}{l}\therefore (120+x)\times 10=0.2\times {10}^3\times 10\\ \Rightarrow 120+x=200\therefore x=80\mathrm{kg}\end{array}$