A cubical block of side 0.5 m floats on water with 30% of its volume under water. What is the maximum weight that can be put on the block without fully submerging it under water? [Take, density of water = 10³ kg/m³]

When a body floats then the weight of the body = upthrust 
$\therefore (50{)}^3\times \frac{30}{100}\times (1)\times \mathrm{g}={\mathrm{M}}_{\text{cube }}\mathrm{g}$       ......(i)  

Let m mass should be placed, then
$(50{)}^3\times (1)\times \mathrm{g}=\left({\mathrm{M}}_{\text{cube }}+\mathrm{m}\right)\mathrm{g}$           ......(ii)

Subtracting equation (i) from equation (ii), we get
$\Rightarrow \mathrm{mg}=(50{)}^3\times \mathrm{g}(1-0.3)=125\times 0.7\times {10}^3\mathrm{g}$

$\Rightarrow \mathrm{m}=87.5\mathrm{kg}$