If you are traveling at 90KTs (groundspeed) it will take you 2/3 of the distance in minutes to reach your destination. For example, if you need to divert for some reason and your new destination is 36 NM away it will take you 24 minutes to reach your new destination if you fly the plane at 90 KTs (groundspeed). This in an important calculation for PPL and CPL flight tests. In reality you will probably just use your GPS, ForeFlight, or Garmin pilot (if you have one) to make the calculations.