Question 220813
Linda and Dave leave simultaneously from the same starting point biking in opposite directions. 
Linda bikes at 5 miles per hour and Dave bikes at 9 miles per hour. 
How long will it be until they ar 28 miles apart from each other?
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You can do this in your head, their relative speed: 9 + 5 = 14 mph
How long to cover 28 at 14 mph, obviously, it's 2 hrs