28. In a large town, Kennedy Avenue is a long north-south avenue with many intersections.

A drunken man is wandering along the avenue and does not really know which way he is going. He is currently at an intersection O somewhere in the middle of the avenue.

Suppose that, at the end of each block, he either goes north with probability 1/2, or he goes south with probability 1/2. Draw a tree diagram to find the probability that, after walking four blocks,

(a) he is back at intersection O;

(b) he is only one block away from intersection O.