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

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08 Jan 2005, 14:37

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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?

The average speed of a car decreased by 3 mph every [#permalink]

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29 Oct 2005, 09:46

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The average speed of a car decreased by 3 miles per hour every successive 8-minute 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

Last edited by Bunuel on 17 Sep 2014, 00:34, edited 1 time in total.

Re: The average speed of a car decreased by 3 mph every [#permalink]

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29 Oct 2005, 18:20

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51

and here is why.
In the last 6th 8 minute interval he travelled 4.8 miles..
so the average speed in the last 8 minute interval is 4.8/8 = 0.6 miles/minute which is he travles 0.6*60 miles in an hour = 36 miles in an hour

Since this was the 6th 8 minute interval and the speed decreases 3 miles/hr every 8 min, backsolvin it.. 51miles/hr

Re: The average speed of a car decreased by 3 mph every [#permalink]

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30 Oct 2005, 05:52

bewakoof wrote:

51

and here is why. In the last 6th 8 minute interval he travelled 4.8 miles.. so the average speed in the last 8 minute interval is 4.8/8 = 0.6 miles/minute which is he travles 0.6*60 miles in an hour = 36 miles in an hour

Since this was the 6th 8 minute interval and the speed decreases 3 miles/hr every 8 min, backsolvin it.. 51miles/hr

Re: The average speed of a car decreased by 3 mph every [#permalink]

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08 Feb 2015, 17:10

GMATT73 wrote:

The average speed of a car decreased by 3 miles per hour every successive 8-minute 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

Assume the speed was X in the first 8-minute interval So the speed would be x-3 after first interval .................... So the speed will be x-15 at the 6th interval

Now,

Rate x Time = Distance

(X-15) x 8 min = 4.8 miles > (x-15) x 8/60 hours = 4.8 miles > X-15 = (4.8) x 60/8= 36 > X=36+15= 51

We're told that a car trip is broken down into 8-minute 'intervals'; this is a little 'strange' since we're used to seeing car trips broken down by distances. The math involved is the same though: we're going to use the Distance Formula and do some arithmetic to get to the correct answer.

We're told that the SIXTH interval has the car traveling 4.8 miles in 8 minutes... We can use this to figure out the speed of the car during that interval:

Distance = (Rate)(Time) 4.8 miles = (R)(8 minutes) 4.8 miles = (R)(8/60 hours) 4.8/(8/60) = R 36 miles/hour = R

Now we know that the car was traveling at an average speed of 36 mph in that SIXTH interval. We're told the average speed DECREASES by 3 miles per hour from one interval to the next, so we can work 'backwards' to figure out the speeds at each of the previous intervals. We're asked for the average speed of the car during the FIRST interval...

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

Hi,

another way DEPENDENT on choices..

the speed in sixth 8-minute interval will be a multiple of 3.. 4.8/6*60..and the reduction in speed was also in multiple of 3.. so the speed in every interval should be multiple of 3.. ONLY D and C are left...

Now the multiples of 3 are alternately ODD and EVEN... so SIX-interval means 5 minuses.. so EVEN multiple of 3 - ODD(5) multiple of 3.. so ans should be ODD multiple of 3.. ONLY D is left D
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Re: The average speed of a car decreased by 3 mph every [#permalink]

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11 Nov 2017, 00:34

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