Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 29 May 2017, 16:47

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# DS- Average rate (m09q15)

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
CEO
Joined: 29 Mar 2007
Posts: 2562
Followers: 21

Kudos [?]: 453 [0], given: 0

DS- Average rate (m09q15) [#permalink]

### Show Tags

29 Nov 2007, 14:29
2
This post was
BOOKMARKED
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

In the morning, John drove to his mother's house in the village at an average speed of 60 kmh. When he was going back to town in the evening, he drove more cautiously and his speed was lower. If John went the same distance in the evening as in the morning, what was John's average speed for the entire trip?

1. In the evening, John drove at a constant speed of 40 kmh.
2. John's morning drive lasted 2 hours.

[Reveal] Spoiler: OA
A

Source: GMAT Club Tests - hardest GMAT questions

I got the answer to this, I just want to see what you guys calculate the answer to be. I come up with a number too large so I wanna see where my calculations are wrong. Thx.
CEO
Joined: 17 Nov 2007
Posts: 3585
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Followers: 576

Kudos [?]: 3989 [2] , given: 360

### Show Tags

29 Nov 2007, 14:44
2
KUDOS
Expert's post
A.

1. Vm=60, Ve=40

Va=2L/t=2L/(tm+te)=2L/(L/Vm+L/Ve)=2VmVe/(Vm+Ve)=2*40*60/100=48kmh
SUFF.

2) John's morning drive lasted 2 hrs - information about evening trip absents.
INSUFF.
CEO
Joined: 29 Mar 2007
Posts: 2562
Followers: 21

Kudos [?]: 453 [0], given: 0

### Show Tags

29 Nov 2007, 15:47
walker wrote:
A.

1. Vm=60, Ve=40

Va=2L/t=2L/(tm+te)=2L/(L/Vm+L/Ve)=2VmVe/(Vm+Ve)=2*40*60/100=48kmh
SUFF.

2) John's morning drive lasted 2 hrs - information about evening trip absents.
INSUFF.

Ok ya I got 48 as well. My notes were so sloppy that I it looked like the time was 48hours... so i was like what the heck is goin on??? owell still got the answer.

Thx walk.
SVP
Joined: 28 Dec 2005
Posts: 1561
Followers: 3

Kudos [?]: 160 [0], given: 2

### Show Tags

15 Dec 2007, 19:03
didnt actually solve for the value, but got A

We can express the overall equation for avg speed just in terms of one variable, which is the total distance. Youll see that this variable cancels out anyways, and once you have the speed for the 2nd leg of the trip, youre good.
Director
Joined: 21 Dec 2009
Posts: 585
Concentration: Entrepreneurship, Finance
Followers: 18

Kudos [?]: 705 [1] , given: 20

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

08 Apr 2010, 18:53
1
KUDOS
distance = speed x time
(1) 60x = 40y................................(1) x & y are corresponding times.
we need (60x + 40y/(x+y)..........(2)
Suff.

(2)60x2 = 120km. Nothing about evening time or speed.
Insuff
A is it.
_________________

KUDOS me if you feel my contribution has helped you.

Manager
Joined: 05 Mar 2010
Posts: 217
Followers: 1

Kudos [?]: 32 [1] , given: 8

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

09 Apr 2010, 01:12
1
KUDOS
We just need the speed at which john travelled in morning and evening to find the his average speed
(2*60*40)/60+40 = 48km/hr
Sufficient

Stmt 2 is not sufficient - no information regarding evening travel

IMO A
_________________

Success is my Destiny

Intern
Joined: 29 Mar 2010
Posts: 43
Followers: 0

Kudos [?]: 11 [1] , given: 1

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

11 Apr 2010, 22:01
1
KUDOS
Just Started my prep.

Was thinking when the average speeds needs to be calculated, from basic logic you need the average speed in the morning and average speed in the evening OR if you dont have that information you need to calculate.

1) provides me that information ( I dont need to calculate anything, calculation seems more like a trap for this questions to waste time).
2) distance in this case has no relevance since the problem was solved in step 1, step 2 only needs to be analyzed for checking if this option contributes to statement 1 or independently resolves the question asked.

My 2 cents.

By the way the calculation seems interesting :

why ((2)*60*40)/60+40 = 48km/hr
wouldn't the average be 60+40/2 we already have from questions and statement 1 the average speed.
_________________

-------------------------------------------------------------------------
Ros.
Nice Post + Some help + Lucid solution = Kudos

The greatest pleasure in life is doing what people say you cannot do | Great minds discuss ideas, average minds discuss events, small minds discuss people.
-------------------------------------------------------------------------

SVP
Joined: 16 Nov 2010
Posts: 1666
Location: United States (IN)
Concentration: Strategy, Technology
Followers: 34

Kudos [?]: 533 [0], given: 36

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

12 Apr 2011, 07:12
(1)
x = distance
Average Speed = 2x/(x/60 + x/40)

Sufficient

(2)

The speed during return trip is not given, so insufficient.

_________________

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

GMAT Club Premium Membership - big benefits and savings

Manager
Joined: 31 Jan 2011
Posts: 105
Followers: 5

Kudos [?]: 69 [0], given: 4

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

12 Apr 2011, 10:30
Answer i got is A

1- Total distance/total time - distance can be assumed and time can be determined using rate - suff
2- Need to know the rate - it cannot be assumed - insuff
Director
Joined: 01 Feb 2011
Posts: 755
Followers: 14

Kudos [?]: 125 [0], given: 42

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

12 Apr 2011, 20:39
1. Sufficient

Avg speed = 2x/((x/40)+(x/60))
2. Not sufficient. Evening trip info missing.

Manager
Joined: 16 Sep 2010
Posts: 220
Location: United States
Concentration: Finance, Real Estate
GMAT 1: 740 Q48 V42
Followers: 9

Kudos [?]: 117 [0], given: 2

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

12 Apr 2011, 21:13
GMATBLACKBELT wrote:
In the morning, John drove to his mother's house in the village at an average speed of 60 kmh. When he was going back to town in the evening, he drove more cautiously and his speed was lower. If John went the same distance in the evening as in the morning, what was John's average speed for the entire trip?

1. In the evening, John drove at a constant speed of 40 kmh.
2. John's morning drive lasted 2 hours.

[Reveal] Spoiler: OA
A

Source: GMAT Club Tests - hardest GMAT questions

I got the answer to this, I just want to see what you guys calculate the answer to be. I come up with a number too large so I wanna see where my calculations are wrong. Thx.

As helpful as it is to see everyone repeat the same equation - I was wondering if someone could specifically walk me through how they arrived at ((2)*60*40)/60+40 = 48km/hr.

I understand that the end result is correct and how since we are using average speeds its irrespective of distance or time. I just am a little confused on the logic behind the equation.

Is the logic something like 2x (because it represents both legs of the journey) x 60km/h (morning speed) x 40km/h (evening speed) / 60+40.

Let me know. Thanks.
Math Forum Moderator
Joined: 20 Dec 2010
Posts: 2013
Followers: 163

Kudos [?]: 1832 [1] , given: 376

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

12 Apr 2011, 23:36
1
KUDOS
u0422811 wrote:
GMATBLACKBELT wrote:
In the morning, John drove to his mother's house in the village at an average speed of 60 kmh. When he was going back to town in the evening, he drove more cautiously and his speed was lower. If John went the same distance in the evening as in the morning, what was John's average speed for the entire trip?

1. In the evening, John drove at a constant speed of 40 kmh.
2. John's morning drive lasted 2 hours.

[Reveal] Spoiler: OA
A

Source: GMAT Club Tests - hardest GMAT questions

I got the answer to this, I just want to see what you guys calculate the answer to be. I come up with a number too large so I wanna see where my calculations are wrong. Thx.

As helpful as it is to see everyone repeat the same equation - I was wondering if someone could specifically walk me through how they arrived at ((2)*60*40)/60+40 = 48km/hr.

I understand that the end result is correct and how since we are using average speeds its irrespective of distance or time. I just am a little confused on the logic behind the equation.

Is the logic something like 2x (because it represents both legs of the journey) x 60km/h (morning speed) x 40km/h (evening speed) / 60+40.

Let me know. Thanks.

$$Average \hspace{2} speed=\frac{Total \hspace{2} Distance}{Total \hspace{2} Time}$$

Average speed for the entire trip needs to be found.

Let the distance of the drive one way be "D"
Distance for the entire trip = D+D=2D

Time taken for the entire trip = Time taken to reach John's Mom's+Time taken for the return journey

$$Time = \frac{Distance}{Speed}$$

Time taken to reach Mom's: $$\frac{D}{60}$$
Time taken for the return trip: $$\frac{D}{40}$$ ---- Using St1

$$Average \hspace{2} speed=\frac{2D}{\frac{D}{60}+\frac{D}{40}}$$
$$Average \hspace{2} speed=\frac{2}{\frac{1}{60}+\frac{1}{40}}$$
$$Average \hspace{2} speed=\frac{2*60*40}{60+40}$$

2.
Using 2, we would know the exact distance "D" as

D= speed*time = 60*2=120km.
But, as we saw earlier, the distance gets canceled from numerator and denominator and won't help us determine the average speed for the entire trip. We need to know how long did it take for both trips.
_________________
Intern
Joined: 16 Jan 2011
Posts: 1
Followers: 0

Kudos [?]: 0 [0], given: 0

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

13 Apr 2011, 09:41
I think 1 is sufficient to answer
Manager
Status: It's "Go" Time.......
Affiliations: N.C.C.
Joined: 22 Feb 2011
Posts: 181
Location: India
Followers: 6

Kudos [?]: 75 [0], given: 2

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

16 Apr 2011, 14:59
Easy one standard equation 2xy/(x+y) will give you the answer!!
_________________

We are twice armed if we fight with faith.

He who knows when he can fight & when He can't will be victorious.

Manager
Joined: 18 Jan 2011
Posts: 230
Followers: 1

Kudos [?]: 37 [0], given: 4

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

17 Apr 2011, 13:15
A, formula 2S1S2/(S1 + S2) S1, S2 being the speeds in each direction
_________________

Good Luck!!!

***Help and be helped!!!****

Intern
Joined: 25 Apr 2010
Posts: 6
Followers: 0

Kudos [?]: 1 [0], given: 0

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

18 Apr 2011, 06:20
Ans A

A. 2*60*40/100 = 48
B. Dist = 60*2 = 120 km; No info on evening's speed. Insufficient
Math Expert
Joined: 02 Sep 2009
Posts: 39062
Followers: 7756

Kudos [?]: 106577 [0], given: 11628

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

17 Apr 2013, 05:12
In the morning, John drove to his mother's house in the village at an average speed of 60 kilometers per hour. When he was going back to town in the evening, he drove more cautiously and his speed was lower. If John went the same distance in the evening as in the morning, what was John's average speed for the entire trip?

The average speed equals to $$\frac{total \ distance}{total \ time}$$.

Say the distance between the town and the village is $$d$$ kilometers, and the speed from the village to town is $$x$$ kilometers per hour, then $$\frac{total \ distance}{total \ time}=\frac{d+d}{\frac{d}{60}+\frac{d}{x}}$$ --> $$d$$ can be reduced, and we get $$speed=\frac{2}{\frac{1}{60}+\frac{1}{x}}$$. So, as you can see we only need to find the average speed from the village to town.

(1) In the evening, John drove at a constant speed of 40 kilometers per hour. Sufficient.

(2) John's morning drive lasted 2 hours. We know nothing about his evening drive. Not sufficient.

_________________
Intern
Joined: 04 Apr 2013
Posts: 16
Concentration: Finance, Economics
GPA: 3
Followers: 0

Kudos [?]: 0 [0], given: 0

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

17 Apr 2013, 05:20
No need to solve anything, that's the point of these questions: all you need to know is if (1) or (2) or neither are sufficient. Its obvious just by first pass that (1) is enough. Given average speeds for entire trip you can get the average speed. Length of trip is not relevant.

Senior Manager
Joined: 16 Dec 2011
Posts: 432
Followers: 11

Kudos [?]: 214 [0], given: 70

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

17 Apr 2013, 06:20
1) As the onward and return journey covered same distance and we know speed for both onward and return journey, we can calculate average speed. Sufficient.

2) There is no information on speed or time taken in the return journey. Not sufficient.

Manager
Joined: 15 Aug 2012
Posts: 110
Location: India
Concentration: Technology, Strategy
Schools: Merage '15 (A)
GPA: 3.6
WE: Consulting (Computer Software)
Followers: 6

Kudos [?]: 47 [0], given: 22

Re: DS- Average rate (m09q15) [#permalink]

### Show Tags

18 Apr 2013, 01:00
In the morning, John drove to his mother's house in the village at an average speed of 60 kmh. When he was going back to town in the evening, he drove more cautiously and his speed was lower. If John went the same distance in the evening as in the morning, what was John's average speed for the entire trip?

1. In the evening, John drove at a constant speed of 40 kmh.
2. John's morning drive lasted 2 hours.

Let distance be x
Case 1:
u1 = 60, t1 = x/60
u2 = 40, t2 = x/40

Hence we calculate avg speed => total dist/total time and get an integer value

SUFFICIENT

Case 2
u2 is unknowd so we cannot get the exact avg value.
INSUFFICIENT

Hence A
Re: DS- Average rate (m09q15)   [#permalink] 18 Apr 2013, 01:00
Similar topics Replies Last post
Similar
Topics:
DS q 1 07 Jun 2011, 10:51
DS problem 1 20 Jan 2010, 04:44
Average speed 3 07 Feb 2013, 01:18
9 DS - Water evaporates at the rate of two liters (m07q26) 29 27 May 2014, 07:07
15 ds rate (m08q34) 20 18 Nov 2011, 01:41
Display posts from previous: Sort by

# DS- Average rate (m09q15)

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

Moderator: Bunuel

 Powered by phpBB © phpBB Group and phpBB SEO Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.