



 
Hi Catrina, 2.6 inches for the diameter of a car wheel seems unreasonable. I think it should be 2.6 feet. There are 5,280 feet in a mile and 60 minutes in an hour so going 75 miles in an hour is equivalent to going $75 \times 5,280$ feet in 60 minutes. hence the speed of the car is \[\frac{75 \times 5,280}{60} = 25 \times 264 \mbox{ feet per minute.}\] The circumference of a circle is $\pi$ times the diameter so the circumference of the wheel is $\pi \times 2.6$ feet. Hence the car moves $\pi \times 2.6$ feet in one revolution. Thus the speed of the wheel is \[\frac{75 \times 5,280}{\pi \times 2.6}\] and the units are \[\frac{\mbox{ feet / minute }}{\mbox{ feet / revolution }} = \frac{\mbox{ revolutions }}{ \mbox{ minute}}.\] Thus the speed of the wheel is \[\frac{75 \times 5,280}{\pi \times 2.6} \frac{\mbox{ revolutions }}{ \mbox{ minute}}.\] How many radians are there in one revolution? Penny 



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