680 miles per hour going north
620 miles per hour going south
required: time at 3900 miles apart
we know that the plane flight at there opposite ways ..the first plane going to north and the second plane going to south but in 1 hour they took about 1300 miles apart therefore if they took 3900 miles apart they reach 3 hours.
680miles + 620miles/hour = 3900miles
hour = 3900miles/1300miles
time = 3 hours.