Let us first analyse if these blocks move together.
For that we will find the acceleration of the total mass (40+10) kg first.
Using formula F=ma or a=Fm=100N50kg.
The acceleration is 2m/s2 and the maximum frictional force =μs×N
=μs×m1g
=(0.60)(10)(9.8)=58.8N
Thus, we see frictional force is smaller than the applied force and proves that the block and the slab do not move relative to each other.
For the block we have F=μmg
=0.4×10×9.8=39.2N
Resulting acceleration of the slab , a=F/m=39.2/40=0.98m/s2
Thus, the resulting acceleration of the slab is 0.98m/s2