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Word Problems/Fred and Frank Problem


saladgirl70 wrote at 2012-11-29 16:03:04
The algebra is sound, but I disagree with the average rate of speed.  Average speed is (Total Distance)/(Total Time).  Frank's average speed is (walk speed + run speed)/2, while Fred's will be slower since he spends more time walking than running.  To see this, try it with the following numbers:  W=5 mph, R=10 mph, D=10 miles.  Then Frank's time = 20/(15)= 1.33 hr.  So his avg speed = 10 mi / 1.33 hr = 7.5 mph = (R + W)/2.  While Fred's time = 1.5 hr.  His avg speed = distance / total time = 10 mi / 1.5 hr = 6.67 mi/hr.  

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