its not asking for the average speed. its asking for the speed for the second half of the trip to be averaged with the first half of the trip to complete the trip at an average of 30 mph. An average of 30 mph means you you complete the ENTIRE trip in 4 minutes. it takes you 4 minutes to do the first half, so there is no time left to do the second part.
Oh, I get it. It’s a trick question. The ‘length’ is artificially divided in ‘half’, limiting the speed during that first half so that it takes at least 4 minutes.
Then both miles are lumped together and the time is used to calculate AVE MPH.
I understand that AVE MPH is based on distance per time, so I see how the 15 mph limit ensures that there is not a possible answer.
However, I don’t like rigged tests. SO.... here’s my answer (I have my own ‘tricks’)
Take the car to a Custom Auto shop and have nitrous installed. Don’t tell the guys who are timing you. Once you head off up the hill, engage the nitrous. This will give you plenty of speed to ensure you can go fast enough on the downhill side to make it there in four minutes.