Question

Two jets leave an airport at noon, one flying north at r mi/h, the other flying south at twice that speed. After 3h how far apart are the planes

Answer 1

The speed of one jet is ' r ' miles per hour, and the speed of the other one
is ' 2r ' miles per hour.  They're flying exactly away from each other, so the
space between them is increasing at the rate of  ' 3r ' miles per hour.

After 3 hours, the distance between them is

                   (3r miles/hour) x (3 hours) = 9r miles.