Rachel E.

asked • 01/12/17

Word Problem

Kari just got a new car with tires that are 21 inches in diameter. If her wheels are spinning at a rate of 800 revolutions per minute, how fast is she going in miles per hour? Round the answer to the nearest whole number.

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Mark M. answered • 01/12/17

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[800rev/min][60min/1hr][2π(21)in/1rev][1ft/12in][1mile/5280ft]
 
  = 100mi/hr 
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Rachel E.

So you're multiplying all of those together?
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01/12/17

Mark M.

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Yes.  Multiply all of the numerators together and divide by the product of all the denominators.  Notice how some of the units cancel as you move from left to right, so that all that remains at the end is miles in the numerator and hours in the denominator.  
 
Mark M (Bayport, NY)
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01/12/17

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