The answer is not 45 mph (with the idea that 15 + 45 / 2 = 30). In that event the man would cover 2 miles in 1/15 + 1/45 of an hour, for an average speed of only 22.5 mph.

In fact he can’t possibly drive fast enough. To average 30 miles per hour, he would have to cover the 2 miles in 1/15 hour, or 4 minutes. But he’s already consumed 4 minutes in driving to the top of the hill (1 mile / 15 miles per hour = 1/15 hour, or 4 minutes). So it can’t be done.