Question 106622
You rent a mountain bike & helmet for $10/hr; if you have your own helmet, $7/hr; or you can buy a helmet for $28. How many hours do you need to use the trail to justify buying the helmet? 
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Let "x" be the number of hours on the trail.
1st: If you buy your own helmet Total Cost = (28 + 7x) dollars
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2nd: If you rent helmet and trail time: Total Cost = 10x
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INEQUALITy:
Buy helmet cost < Rent helmet
28+7x < 10x 
3x >28
x > 9 1/3 hours.
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You have it right.
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Cheers,
Stan H.