And just where does it state that the trip has to be completed in four minutes?
It’s in the question itself. You must average 30mph the whole trip.
30mph is 2 miles per minute. The whole trip is two miles therefore at 15mph going up the hill it’ll take the entire 4 minutes doing the first uphill mile. There is no time left to do the second mile. It’d have to be a “beam me up Scotty” moment.
Q1- Distance, Time, and Speed An old car has to travel a 2-mile route, uphill and down. Because it is so old, the car can climb the first mile—the ascent—no faster than an average speed of 15 mi/h. How fast does the car have to travel the second mile—on the descent it can go faster, of course—to achieve an average speed of 30 mi/h for the trip?
There's the question. Doesn't state that the trip must take 4 minutes. The only place that was stated was in someone else's comment. And that person is wrong.
You must average 30mph the whole trip.
So... how does one determine average MPH for such a course?
If you drive 1 mile at 15 mph, and 1 mile at 45 mph, you will have gone 2 miles, and your average MPH is the two SPEEDS ADDED and then DIVIDED by the length (2). That gives 30 MPH average for a course length of 2 miles.
Doesn't matter how darn long it took. It isn't an issue in the question. There was no time limitation specified.