Find out the range of force for which smaller block is at rest with respect to bigger block.

Explanation

Smaller block is at rest w.r.t. the bigger block . Let both the block travel together with acceleration a F.B.D. of smaller block w.r.t. to the bigger block.
`f_("max") = mu xx N `
`N = ma`
`f = mu ma ` …(1)

`rArr f = mg ` ...(2)
from ( 1) & (2)
`a = g//mu = 20 m//s^(2)`
SO `F = 20( M + m ) = 20 ( 5) = 100N`
If `F ge 100N` Both will travel together