(b)
When the plane moves east, the net velocity of the plane in air is:

Now, the time to reach its destination is:

When the plane moves west, the net velocity of the plane in air is:

Now, the time to reach its destination is:

The total time taken by the plane to complete the trip is:


(c)
The speed of the plane in air, when the air moves North or South is:


The speed of the plane remains the same on its round trip, and therefore the total time taken by the plane is:


(d)
It is important to note that in parts (b) and (c) one must assume that v > u or else the time t will be negative and imaginary correspondingly. Therefore, the plane may get to its destination in one direction, but it cannot fly back in real time.