Two blocks A and B of masses $m_A=1kg$ and $m_B=3kg$ are kept on table as shown in figure. The coefficient of friction between A and B is 0.2 and between B and surface of table also 0.2.  The maximum force F (in N) can be applied on B horizontally, so that the block A does not slide over the block B is $2x$. The value of $x$ is (take $g=10m/s^2)$