Answer
0.27 kg.
Work Step by Step
The buoyant force on the completely submerged wood equals the combined weight of the wood and the copper.
$$(m_{wood}+m_{Cu})g=V_{wood}\rho_{water}g= \frac{m_{wood}}{ \rho_{wood}}\rho_{water}g $$
$$ m_{Cu}= m_{wood}(\frac{\rho_{water}}{ \rho_{wood}}-1)=0.27\;kg$$