An air traffic controller is monitoring two small planes flying at the same altitude. One is flying towards the control tower from the west at
120 miles per hour and is currently 5 miles away from the tower. The
other plane is now on top of the tower and flying south at 150 miles per
hour. In order to respect local safety rules, the planes have to be always
more than 4 miles from each other or fly at different altitudes. Should
the controller order one of the planes to change altitude, or will they
always be farther than 4 miles from each other?