From Mechanics

See diagram.

\displaystyle R1 is the reaction force holding up the combined weights of the two lighter boxes. Equilibrium of the two boxes gives

\displaystyle \begin{align} & \uparrow \ R1-(5+10)g=0 \\ & \text{giving}\ R1=15\times 9\textrm{.}8=147\ \text{N} \\ \end{align}

The weight is \displaystyle mg=12\times 9\textrm{.}8=117\textrm{.}6\ \text{N}

Finally the box is in equilibrium

\displaystyle \begin{align} & \uparrow \ R2-R1-mg=0 \\ & \text{giving}\ R2=147+117\textrm{.}6=264\textrm{.}6\ \text{N} \\ \end{align}