Two blocks, A and B, are stacked.  Block B is attached to a wall by an inextensible string.  A force of 25 lb is applied to Block B, which is enough to make it slide to the left. If $$mu_k=0.4$$ between A and B, and if $$mu_k=0.6$$ between B and the floor, the acceleration of B $$(ft//s^2)$$ is most nearly:

We need to determine the horizontal forces applied on block B by both A and the floor. To do this, we just need to find the normal force applied by the floor on the combined mass of block A and B, and also the normal force of block B on A.

The normal force exerted on block B is 6lb, therefore the force due to friction is $$0.6 * 6lb = 3.6lb$$

The normal force exerted on block A is 3lb, therefore the force due to friction is $$0.4*3lb = 1.2lb$$

The total force exerted on block B as it's pulled is $$25-3.6-1.2 = 20.2lb$$

Now use F = ma to find the acceleration of block B.  

$$m_b = (3lb)/(32.2 (ft)/s^2) = 0.093text( slugs)$$

Finally:

$$a = F/m ~~ 20.2/0.093 ~~ 217.2 (ft)/s^2$$