If you drive an average rate of 40 mph will it take you two hours to drive 20 miles across town?

No. It would only take you half of an hour. 40 miles per hour literally means that in an hour, you would have traveled 40 miles. If that's the case, then to travel 20 miles (half of 40) at a rate of 40 mph would only take half of an hour

It would take 30 minutes to drive across town. In one hour if you can drive 40 miles, then it would only take half of the hour to drive half the distance, so the answer is 30 minutes.