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A bus from city M is traveling to city N at a constant speed [#permalink]
07 Nov 2009, 02:23

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00:00

A

B

C

D

E

Difficulty:

95% (hard)

Question Stats:

36% (03:22) correct
64% (02:16) wrong based on 250 sessions

A bus from city M is traveling to city N at a constant speed while another bus is making the same journey in the opposite direction at the same constant speed. They meet in point P after driving for 2 hours. The following day the buses do the return trip at the same constant speed. One bus is delayed 24 minutes and the other leaves 36 minutes earlier. If they meet 24 miles from point P, what is the distance between the two cities?

A. 48 B. 72 C. 96 D. 120 E. 192

The source is GMATClub's diagnostic test... would look forward to see some innovative approach to this. Thanks!

Re: two buses, same speed... head spinning [#permalink]
07 Nov 2009, 03:13

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Hmmm I did this question a while ago and got it wrong so trying again

Rate x time = distance. For the initial trip lets the distance to the midpoint be represented by P. EQN1: R x 2 = P

For the second trip we know one bus left late and one left early. Together this is just a tricky way of saying one bus left an hour after the other. We know the total trip takes 4 hours (since getting to P is 2 hours). The second trip can be represented by:

Since the trip takes 4 hours if a bus leaves one hour early, the reminaining 3 hours are split between the two buses, ie. 1 + 3/2 = 2.5 EQN2: R x 2.5 = P + 24

EQN2-EQN1 : 0.5R=24 R=48

So the distance is rate x time = 48 x 4 = 192. ANS = E

Re: two buses, same speed... head spinning [#permalink]
07 Nov 2009, 03:35

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Normally I goof up with distance-speed probs, but got this one right.

here, B1 and B2 are coming towards each other at the same speed, so when they meet they MUST have traveled the same distance. in 2 hours B1 has traveled 2S miles ( let S be the speed ) so, B2 has also traveled 2S miles

So we have a line segment MN having a distance 4S and PM=MN = 2S. Divide this line segment into four parts with each having distance S.

Okay, for the second trip the total time delay is 60 mins=1 hour. Suppose B2(at N) starts early. In this one hour B2 has traveled S miles. At this time B1 will start and both buses will meet at a point which is halfway of the remaining distance(3S) = 3S/2.

Re: two buses, same speed... head spinning [#permalink]
07 Nov 2009, 05:45

I am not sure if I am right, but my ans differs from the 2 posted earlier. I think the distance between M and N should be 96.

Facts given: Both buses travel at the same speed. When they leave at exactly same time from M and N respectively, they meet at P in 2 hours. This means the total journey should be taking 4 hours for both of them individually.

Now, one bus leaves 36 min earlier and other one is delayed by 24 min, there is a total gap of 60 min or 1 hour between the two buses. If their meeting point is now shifted by 24 miles, it means their respective speeds are 24 miles an hour. This drives me to a conclusion that the totla distance between M and N should be 4 * 24 = 96 miles.

Re: two buses, same speed... head spinning [#permalink]
07 Nov 2009, 07:45

5

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kalpeshchopada7 wrote:

I am not sure if I am right, but my ans differs from the 2 posted earlier. I think the distance between M and N should be 96.

Facts given: Both buses travel at the same speed. When they leave at exactly same time from M and N respectively, they meet at P in 2 hours. This means the total journey should be taking 4 hours for both of them individually.

Now, one bus leaves 36 min earlier and other one is delayed by 24 min, there is a total gap of 60 min or 1 hour between the two buses. If their meeting point is now shifted by 24 miles, it means their respective speeds are 24 miles an hour. This drives me to a conclusion that the totla distance between M and N should be 4 * 24 = 96 miles.

Please correct me if i'm wrong!!

Dear Economist and yangsta8, thanks for the new approaches... i still am spinning though kalpesh, following is the official explanation which I spent some time to understand (24miles in 30mins):

(I didn't know that these questions have their own thread, sorry for repost) gmat-diagnostic-test-question-79354.html The buses travel at the same constant speed. It would take one bus to travel 4 hours to cover the distance between the cities M and N (two buses drove for 2 hours each). We need to find the speed of the bus. If the first bus was delayed by 24 minutes and the second one left 36 minutes earlier, it makes the second bus 24+36=60 minutes ahead of the first bus.

The meeting point was 24 miles away on this second day. We know the distance difference between the two meeting points, but we also need to find difference in time those 24 miles were covered. If the second bus drove for 1 hour before the first one departed, each of them had to go for another 1.5 hour to meet (1.5 hour + 1.5 hour + 1 hour). The second bus traveled for 2.5 hours and the first one for 1.5 hour. Therefore the meeting point on the second day was 30 minutes away from that of the previous day.

So the second bus covered 24 miles in 30 minutes, which gives us the speed of the bus, 48 mph. We can calculate the distance as we already know the speed:

Re: two buses, same speed... head spinning [#permalink]
07 Nov 2009, 08:02

20

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Expert's post

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A bus from city M is traveling to city N at a constant speed while another bus is making the same journey in the opposite direction at the same constant speed. They meet in point P after driving for 2 hours. The following day the buses do the return trip at the same constant speed. One bus is delayed 24 minutes and the other leaves 36 minutes earlier. If they meet 24 miles from point P, what is the distance between the two cities?

A. 48 B. 72 C. 96 D. 120 E. 192

Distance between the cities d.

First meeting point \frac{d}{2}, as both buses travel at the same constant speed and leave the cities same time they meet at the halfway.

Total time to cover the d 4 hours, as the buses meet in 2 hours.

On the second day first bus traveled alone 1 hour (36min +24min), hence covered 0.25d, and 0.75d is left cover.

They meet again at the halfway of 0.75d, which is 24 miles from \frac{d}{2}:

Re: two buses, same speed... head spinning [#permalink]
29 Mar 2011, 20:41

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M----------x------ P-----(d-x)--------N

2v + 2v = d

x = 2v (they meet at halfway)

Next day they meet at x - 24 miles from city M

d/2v = 2 -> On day 1

Next day, in 1 hr, B2 travels v distance = d/4 miles, and then it travels (24 miles + d/4) from there for t hrs at speed v where it meets B1 (B1 has also traveled for t hrs by then)

(24+ d/4)/v = (d/2-24)/v

=> 48 = d/2 - d/4

=> d = 4 * 48 = 192

Answer - E _________________

Formula of Life -> Achievement/Potential = k * Happiness (where k is a constant)

Re: two buses, same speed... head spinning [#permalink]
27 Oct 2011, 05:15

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Expert's post

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gmattokyo wrote:

A bus from city M is traveling to city N at a constant speed while another bus is making the same journey in the opposite direction at the same constant speed. They meet in point P after driving for 2 hours. The following day the buses do the return trip at the same constant speed. One bus is delayed 24 minutes and the other leaves 36 minutes earlier. If they meet 24 miles from point P, what is the distance between the two cities?

* 48 * 72 * 96 * 120 * 192

The source is GMATClub's diagnostic test... would look forward to see some innovative approach to this. Thanks!

A little bit of visualization can help you solve this question in a moment. Look at the diagram below. Since they travel at same speed for 2 hrs, point P must be right in the middle of M and N.

Attachment:

Ques3.jpg [ 13.29 KiB | Viewed 6895 times ]

On the return trip, the following is what happens. One bus starts at 11:24 and reaches P at 1:24. The other bus starts at 12:24 and reaches mid way between P and N at 1:24 (in one hour, it covers half of the distance it usually covers in 2 hrs). At 1:24, both buses have a fourth of the total distance between them. They will both cover equal distances and meet at a point which is 1/8th of the total distance from P. This distance is given as 24 miles. Hence total distance = 24*8 = 192 miles _________________

Re: two buses, same speed... head spinning [#permalink]
27 Oct 2011, 11:33

1

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@Bunuel - Very neat explanation. Kudos to you..!! _________________

----------------------------------------------------------------------------------------- What you do TODAY is important because you're exchanging a day of your life for it! -----------------------------------------------------------------------------------------

Re: two buses, same speed... head spinning [#permalink]
09 Dec 2011, 03:39

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Fro those of you who are dependent on MGMAT RTD chart (as i am) can try the following: From the attached chart#2 P=R(T+3/5)-24 P=R(T-2/5)+24 Therefore, R(T+3/5)-24=R(T-2/5)+24 Solving for R, we get Rate=48 m/h Plug in value in Chart#1: p=96 So, D=192

Re: two buses, same speed... head spinning [#permalink]
27 Feb 2012, 22:36

Expert's post

T740qc wrote:

Bunuel wrote:

\frac{d}{2}-24=\frac{0.75d}{2}

why do you assume -24 instead of +24?

You can derive this either my reasoning or simply by noticing that d/2>0.75d/2, so it should be d/2-24=0.75d/2 (greater value minus 24 equals to smaller value).

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