Two blocks ($m=0.5 \mathrm{kg}$ and $M=4.5 \mathrm{kg}$) are arranged on a horizontal frictionless table as shown in the figure. The coefficient of static friction between the two blocks is $\frac{3}{7}.$ Then the maximum horizontal force that can be applied on the larger block so that the blocks move together is $N.$ (Round off to the Nearest Integer) [Take $g$ as $9.8 \mathrm{m} {\mathrm{s}}^{-2}$]

Hence, the correct answer is 21.