At 15 miles per hour, it took you 20/15 hours, or 1 hour, 20 minutes to get from A to B. For the overall speed to be 30 miles per hour, you'd have to cover the 40 miles in 40/30 hours, or that same 1 hour, 20 minutes. But you've used up that time already.
So, it would be impossible to obtain an overall speed of 30 miles per hour, unless you somehow magically went from B to A instantaneously. |