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# What was John's average driving speed in miles per hour

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Director
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What was John's average driving speed in miles per hour [#permalink]

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08 Oct 2005, 09:00
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What was John's average driving speed in miles per hour during a 15-minute interval?

(1) He drove 10 miles during this interval.
(2) His maximum speed was 50 miles per hour and his minimum speed was 35 miles per hour during this interval.
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Cheers, Rahul.

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08 Oct 2005, 09:46
A it is...

use the Distance=speed*time

we have time, we have distance, we can calcuate speed....

II is insuff we dont know the distance...
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08 Oct 2005, 10:33
A

B - We need to know the distance.
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08 Oct 2005, 20:17
The way I worked this out was -

As per B,
In 15 minutes, the maximum speed is 50/4 miles and minimum speed is 35/4 miles.
So we have 2 speeds, and we can calculate the average speed by using the relation Average speed is the ration of twice the product of two speeds to the sum of two speeds.
Va = 2V1V1/V1+V2

Where am i going wrong - Oh oh....we can use this when the distance is constant! $#&$# I messed it up!
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08 Oct 2005, 20:21
rahulraao wrote:
The way I worked this out was -

As per B,
In 15 minutes, the maximum speed is 50/4 miles and minimum speed is 35/4 miles.
So we have 2 speeds, and we can calculate the average speed by using the relation Average speed is the ration of twice the product of two speeds to the sum of two speeds.
Va = 2V1V1/V1+V2

Where am i going wrong - Oh oh....we can use this when the distance is constant! $#&$# I messed it up!

these are two extrem speeds. there could be any rate of speeds in between these two speeds. so we cannot average these speeds.

we could have averaged these two extream speeds, if each of the speed had maintained for half of the time interval time.....
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