A boy of mass $4 \mathrm{kg}$ is standing on a piece of wood having mass $5 \mathrm{kg}$. If the coefficient of friction between the wood and the floor is $0.5 ,$ the maximum force that the boy can exert on the rope so that the piece of wood does not move from its place is _______ $\mathrm{N}$. (Round off to the Nearest Integer)

[Take $\left.g=10{ \mathrm{m} \mathrm{s}}^{-2}\right]$

Hence, the correct answer is 30.