Two blocks $m_1=2kg$ and $m_2=4kg$ are connected with a light string which runs over a frictionless peg to a hanging block of mass M as shown in figure. The coefficient of sliding friction b/w block $m_1$ and the horizontal surface at the speeds involved is  ${\mu }_1=0.2$.The coefficient of friction b/w the blocks is ${\mu }_2=0.4$. For block  $m_2$ to not slip on block $m_1$ when it slides over the surface, the mass $M$ of the hanging block is (in Kg)