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# The average speed of a car decreased by 3 miles per hour

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Manager
Joined: 19 Sep 2003
Posts: 158
Location: michigan
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The average speed of a car decreased by 3 miles per hour [#permalink]  08 Jan 2005, 13:37
The average speed of a car decreased by 3 miles per hour every successive 8-minutes interval. If the car traveled 4.8 miles in the sixth 8-minute interval, what was the average speed of the car, in miles per hour, in the first 8 minute interval?

A.35
B.40
C.48
D.51
E.53
Manager
Joined: 14 Dec 2004
Posts: 119
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Kudos [?]: 10 [0], given: 0

rate [#permalink]  08 Jan 2005, 14:32
In the 6th interval, r=d/t = (4.8)*(15/2)=36 mph

Since the rate decreased by 3 mph in each successive interval, in the first it would have been r=51mph.
Manager
Joined: 19 Sep 2003
Posts: 158
Location: michigan
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Kudos [?]: 0 [0], given: 0

Didn't get it [#permalink]  08 Jan 2005, 14:54
Hello cloaked_vessel:

You got the right answer, but I am not sure how you got to your setup.

THX,

wiseguy
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Joined: 07 Jun 2004
Posts: 614
Location: PA
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Kudos [?]: 270 [0], given: 22

Let x be the speed at time inteeval 1 so it decreases by 3mph so at 6th its speed is x-15

Now in the time span of 8 min it travles 4.8 miles so

speed = 4.8 / ( 8 / 60 ) = 36 mph

But x - 15 = 36

x = 51
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