



 
Hi Ron, The units give the key here. You divided 3600 by 5280 to get 0.68181818 and the units are \[\frac{\mbox{sec}}{\mbox{hr}}\times \frac{\mbox{miles}}{\mbox{feet}}.\] To get 34 you multiplied 0.68181818 by 50 so the units now are \[\frac{\mbox{sec}}{\mbox{hr}}\times \frac{\mbox{miles}}{\mbox{feet}}\times \mbox{feet}\] which simplifies to \[\frac{\mbox{sec}\times \mbox{miles}}{hr}.\] But you want miles per hour. I would let the units guide me from the start. the vehicle travels 50 feet in 2.5 seconds so that's \[\frac{50}{2.5} \frac{\mbox{feet}}{\mbox{sec}}.\] To convert to miles per hour you need \[\frac{50}{2.5} \frac{\mbox{feet}}{\mbox{sec}} \times \frac{\mbox{__?__}}{\mbox{__?__}} \frac{\mbox{sec}}{\mbox{hr}}\times\frac{\mbox{__?__}}{\mbox{__?__}}\frac{\mbox{miles}}{\mbox{feet}}.\] You know that there are 3600 seconds in an hour and 5280 feet in a mile so the above simplifies to \[\frac{50}{2.5} \times \frac{3600}{1} \times\frac{1}{5280}\frac{\mbox{miles}}{\mbox{hr}}.\] Penny  


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