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If you drive one mile at 30 mph and one mile at 40 mph what is your average speed for the two miles?
While this seems straightforward by taking the average between 30 and 40 (35 mph), the answer is more complex, because LESS TIME is spent driving at the FASTER speed than at the SLOWER speed.
Using only Englishmeasurements, the average speed is computed as the formula Va= (d1+d2)/ (t1 + t2):
1 mile at 30 mph - time is 1/30 hour or 2 minutes.
1 mile at 40 mph - time is 1/40 hour or 1.5 minutes.
Total distance traveled = 2 miles
Total time elapsed = 3.5 minutes
Average speed = 2 miles / (3.5 minutes) = .5714 miles per minute = 34.3 mph
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If the question had been phrased as "one MINUTE at 30 mph and one MINUTE at 40 mph", then the answer would indeed be the obvious average speed of 35 mph.
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