So the first thing we want to do is find out how much distance the faster plane has to make up. So 180 multiplied by 122 is 21960 miles. Then find the difference between the two rates in order to find the rate of catchup. So 450 - 180 = 270 mph. Then simply divide 21960/270 = 81.33 hours approx. (81 + 1/3). Hope this helps.