(a) Volume of log of wood $V = \frac{{{\rm{mass}}}}{{{\rm{density}}}} = \frac{{120}}{{600}}=0.2 m^3$
Let x weight that can be put on the log of wood.
So weight of the body = $(120 + x) \times 10\;N$
Weight of displaced liquid = $V\sigma g = 0.2 \times {10^3} \times 10\;N$
The body will just sink in liquid if the weight of the body will be equal to the weight of displaced liquid.
 $(120 + x) \times 10 = 0.2 \times {10^3} \times 10$
$ \Rightarrow \;120 + x = 200$ 

$x = 80 kg$