Two jets leave an airport at noon, one flying north at r mi/h, and the other flying south at twice that speed. After 3 h, how far apart are the planes.
After 3 hours the one flying North has traveled 3r miles
And the one flying South has traveled 3(2r) miles = 6r miles
So....they are 3r + 6r = 9r miles apart after 3 hours
