The average speed of a car decreased by 3 mph every

Question

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

cloaked_vessel · Answer

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.

wiseguy · Answer

Hello cloaked_vessel:

You got the right answer, but I am not sure how you got to your setup.

Please give me more detail.

THX,


wiseguy

rxs0005 · Answer

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

xennie · Answer

D. 51

av. speed in 6th interval 36mph
+3 +3 +3 +3 +3 = 51mph in 1st 8 minute interval.

bewakoof · Answer

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

mohish · Answer

Hmm...man you are NOT bewakoof :wink:

remgeo · Answer

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.

Salvetor · Answer

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

EMPOWERgmatRichC · Answer

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:  D

GMAT assassins aren't born, they're made,
Rich

Divyadisha · Answer

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

chetan2u · Answer

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

gracie · Answer

4.8 miles/(1/7.5 hours)=36 mph
36+5*3=51 mph

PSKhore · Answer

Speed in the 6th interval of  8 min  = S6

S6 = 4.8/(8/60).....Converting 8 min to hour

S6 = 36 mph

For the sixth interval the speed is s-3*5 since for every interval the speed decreases by 3

s - 15 = 36

s= 51

The average speed of a car decreased by 3 mph every

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The average speed of a car decreased by 3 mph every

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

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