what is the formula for this problem and how do you figure it out

Question

Two airplanes leave the same airport at 8:00 a.m. traveling in opposite directions. The northbound plane is traveling at 450 mph (relative ground speed) and the southbound plane is traveling 350 mph (relative ground speed). At what time will the two planes be 3200 miles apart?

Answer 1

d=r*t

3200=450*t+350*t

3200=800*t

t=3200/800

t=4 hours

4 hours from 8:00 AM is 12:00 Noon

Answer 2

I do not have any formula. However, to calculate how many hours will pass, you need to calculate how many hours will pass as each plain travels given distance per hour. 4 hours