Two planes left an airport at the same time in opposite directions. One plane flies twice as fast as the other. After 4 hours, they are 2,160 miles apart. How fast is each plane traveling?
d = s · t (d - distance in miles, s - speed in miles per hour, t - time in hours)
Let's assume that "s" is the speed of the slower plane, then "2s" is the speed of another one. Because planes are flying in opposite directions, distance between them increases as fast as "s + 2s" mi/hour.
3s · 4 = 2160 ---> 12s = 2160 ---> s = 2160/12 ---> s = 180 mi/h
2s = 360 mi/h