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Wiseguy hope the below explnation will help you

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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Re: The average speed of a car decreased by 3 mph every [#permalink]
D. 51

av. speed in 6th interval 36mph
+3 +3 +3 +3 +3 = 51mph in 1st 8 minute interval.
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Re: The average speed of a car decreased by 3 mph every [#permalink]
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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
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Re: The average speed of a car decreased by 3 mph every [#permalink]
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


Hmm...man you are NOT bewakoof :wink:
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Re: The average speed of a car decreased by 3 mph every [#permalink]
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If Original speed is x, in the 6th interval, speed is x-15
Time for 6th interval = 8min = 8/60 hrs
Distance = 4.8 mph

D=s*t
4.8 = (x-15)8/60.

Solving for x, we get 51 which is original speed.
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Re: The average speed of a car decreased by 3 mph every [#permalink]
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

Ans D
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Re: The average speed of a car decreased by 3 miles per hour [#permalink]
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Hi All,

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...

6th interval = 36mph
5th interval = 39mph
4th interval = 42mph
3rd interval = 45mph
2nd interval = 48mph
1st interval = 51mph

Final Answer:

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Re: The average speed of a car decreased by 3 mph every [#permalink]
Speed in 6th interval= 4.8*60/8= 36 miles/ hr

Assume speed in 1st interval is x miles/hr. Then speed in 6th interval will be x-15 miles/hr

x-15=36
x=51

D is the answer :)
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Re: The average speed of a car decreased by 3 mph every [#permalink]
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wiseguy wrote:
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]
4.8 miles/(1/7.5 hours)=36 mph
36+5*3=51 mph
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Re: The average speed of a car decreased by 3 mph every [#permalink]
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