40 mph would be 120 miles in 3 hours (120/40)
80 mph is 120 miles in 1.5 hours(120/80)
3 - 1.5 = 1.5, not 3 hrs
Change the speed to 20 mph( 1/2 the speed) to double the hours.
20 mph = 6 hours(120/20)
40 mph = 3 hrs(120/40)
6 - 3 = 3 hours difference