The Colliding Missiles

 

TWO MISSILES speed directly toward each other, one at 9,000 miles per hour and

the other at 21,000 miles per hour. They start 1,317 miles apart. Without using

pencil and paper, calculate how far apart they are one minute before they collide.

The answer:

The two missiles approach each other with combined speeds of 30,000 miles

per hour, or 500 miles per minute. By running the scene backward in time we

see that one minute before the collision the missiles would have to be 500 miles

apart.