Friction

goals
Blocks shown in figure moves with constant velocity $10 \ m/s$ towards right. All surfaces is constant are rough. The friction force applied by B on A is:
A $40$ kg box rests on the horizontal floor of a truck which is initially at rest on a horizontal road. The coefficient of kinetic friction between the floor and the box is $0.2$. The truck at time $t=0$ accelerates constantly at $5$ $ms^{-2}$ and the box is $6$ m away from the rear opening of the truck.
A car is moving on a horizontal circular road $0.1 km$ with constant speed. If coefficient of friction between tyres of car and road is $0.4$, then speed of car may be $(g = 10 m/s^2)$
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A $25$ kg body is resting on a horizontal plane having coefficient of friction $0.2$. A force of $200$N is required to pull the body horizontally. The acceleration developed by the body is: