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Old 04-23-2010, 01:20 AM   #10
Mikaiel
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Default Re: Brain Teasers and Riddles

Quote:
Originally Posted by Doomsday Dallas
explain.

You can't do (32+44) / 2 because that would mean you were going at 32 for the exact same time that you were at 44. Let's say you have to drive for 40 miles and you drive the 1st 20 miles at 5 miles/hour and the other 20 miles at 195 miles/hour. Your average speed is gonna be A LOT smaller than 100 miles/hour.

d = distance home-work
t1 = Time between home-work going at 32 miles/hour
t2 = Time between work-home going at 44 miles/hour

speed = distance / time

So :

32 = d / t1
t1 = d / 32

44 = d / t2
t2 = d / 44

x = average speed

x = 2d / (t1 + t2)

x = 2d / (d/32 + d/44)

x = 2d / [d (1/32 + 1/44)]

x = 2 / (1/32 + 1/44)

x = 37.05
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