If it takes 6 hours to complete the trip at 80 miles per hour, that means that 80 times 6 gives us the distance of the trip.
80 times 6 = 480 miles
so the trip is 480 miles in total.
Now that we know this, we can just divide 480 by 120 in order to get the rate for 120mph!
480 divided by 120 is 4!
So that means that it would take 4 hours at a rate of 120 mph to make the trip!