Terry can run 15 miles in the same amount of time it takes mark to run 21 miles. if mark runs 2 mph faster than terry, how fast does mark run?

The amount of time required to travel a certain distance is equal to quotient of distance and speed. We let x be the speed of Terry and Mark's will be x+2. The equation that would best represent the scenario is,
                             15/x = 21/(x+2)
The value of x is 5. Thus, Mark's speed is 7 mph. 

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