Two cities are 2160 miles apart. a plane leaves one of them, traveling towards the other at an average speed of 320 miles per hour. at the same time a plane leaves the other, traveling towards the first, at an average speed of 400 miles per hour. how long will it take them to meet?

Speed=distance /time  ⇒ distance=Speed x time.
A=a city
B=other city.
 

         320 miles/h                                    400 miles/h
         →                            ←

         AB
                                    2160 miles

x=time

We can sugget this equation:
320x+400x=2160
720 x=2160
x=2160/720
x=3

3 hours.