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

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.