Given  $m_A=30kg,m_B=10kg,m_C=20kg$. Between A and B; ${\mu }_1=0.3$ , between B and C ; ${\mu }_2=0.2$and between C and ground ${\mu }_3=0.1$ . The least horizontal force which can start the motion of any part of the system of three blocks resting upon one another as shown in figure is F (in N). Write the value of  $F/12(g=10m/s^2)$