Initially the system is at rest. find out minimum value of F for which sliding starts between the two blocks.

Explanation

At just sliding condition limiting friction is acting.

F – 50 = 20 a ........... (1)
f = 10 a .................... (2)
50 = 10 a
`therefore a=5m//s^(2)`
hence `F=20+20 times 5=150N`
`therefore F_("min")=150N`