Question 361764
Elaine started the season running a 7-minute mile and is reducing her time by 5 seconds a week. 
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E(x) = 420 - 5x where x is number of weeks
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Mark started with a 9-minute mile and is cutting his time by 515 seconds a week. 
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I assume that 515 you posted is wrong.  Maybe you meant 51.5.  If so
M(x) = 540 - 51.5x where x is number of weeks.
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What equation describes Elaine's time in weeks. What single equation expresses the time when the two runners are running a mile at the same pace?
420-5x = 540-51.5x
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Cheers,
Stan H.