It is currently 18 Oct 2017, 11:55

### 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

# Susan drove an average speed of 30 miles per hour for the

Author Message
TAGS:

### Hide Tags

Manager
Joined: 07 Feb 2010
Posts: 155

Kudos [?]: 741 [5], given: 101

Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

02 Nov 2010, 06:56
5
KUDOS
12
This post was
BOOKMARKED
00:00

Difficulty:

5% (low)

Question Stats:

76% (00:36) correct 24% (00:35) wrong based on 857 sessions

### HideShow timer Statistics

Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55
[Reveal] Spoiler: OA

Last edited by Bunuel on 13 Oct 2013, 03:01, edited 1 time in total.
Moved to PS forum.

Kudos [?]: 741 [5], given: 101

Math Expert
Joined: 02 Sep 2009
Posts: 41886

Kudos [?]: 128752 [2], given: 12182

Re: susan drove an average speed [#permalink]

### Show Tags

02 Nov 2010, 07:04
2
KUDOS
Expert's post
10
This post was
BOOKMARKED
anilnandyala wrote:
susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

a 35
b 40
c 45
d 50
e 55

$$average \ speed=\frac{total \ distance \ covered}{total \ time}$$;

Total distance covered = 30 + 30 = 60 miles;
Total time = 30/30 + 30/60 = 3/2 hours;

$$average \ speed=\frac{total \ distance \ covered}{total \ time}=\frac{60}{\frac{3}{2}}=40$$.

_________________

Kudos [?]: 128752 [2], given: 12182

Senior Manager
Joined: 24 Aug 2009
Posts: 499

Kudos [?]: 839 [0], given: 276

Schools: Harvard, Columbia, Stern, Booth, LSB,
Re: Susan drove an average speed of 30 mph for the first 30 min [#permalink]

### Show Tags

19 Mar 2013, 22:05
Your reasoning is correct. Probably the answer is incorrect unless you have not changed either the question or the answer.
_________________

If you like my Question/Explanation or the contribution, Kindly appreciate by pressing KUDOS.
Kudos always maximizes GMATCLUB worth
-Game Theory

If you have any question regarding my post, kindly pm me or else I won't be able to reply

Kudos [?]: 839 [0], given: 276

Manager
Status: Looking to improve
Joined: 15 Jan 2013
Posts: 174

Kudos [?]: 72 [0], given: 65

GMAT 1: 530 Q43 V20
GMAT 2: 560 Q42 V25
GMAT 3: 650 Q48 V31
Re: Susan drove an average speed of 30 mph for the first 30 min [#permalink]

### Show Tags

19 Mar 2013, 22:51
Avg. speed = total distance / total time

Total distance = 60 miles
Total time = 30 / 30 + 30/60 = 3/2

Avg. speed = 40.

_________________

KUDOS is a way to say Thank You

Kudos [?]: 72 [0], given: 65

MBA Section Director
Status: Back to work...
Affiliations: GMAT Club
Joined: 22 Feb 2012
Posts: 4707

Kudos [?]: 3707 [1], given: 2420

Location: India
City: Pune
GMAT 1: 680 Q49 V34
GPA: 3.4
Re: Susan drove an average speed of 30 mph for the first 30 min [#permalink]

### Show Tags

20 Mar 2013, 02:09
1
KUDOS
Expert's post
jsphcal wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. If she made no stops during the trip, what was Susan's average speed, in miles per hour, for the entire trip?

a. 35
b. 40
c. 45
d. 50
e. 55

The Common Formula is Average speed = Total distance / Total time.
However if the distances traveled with two different speeds are the same (as 30 in this case), we can calculate the average speed as per following rule.
ratio of two distinct speeds = 1:2
Difference of two distinct speeds = 60 - 30 = 30
Now divide this difference by the sum of intergers of ratio (i.e. by 1+2 = 3 in this case ) 30/3 = 10
Average speed will be 10 X 1 part away from the lower speed i.e. 30 + 10 X 1 = 40.

The benefit of this method is once practiced enough you can calculate the average speed mentally or with minimal calculation and thereby can save valuable time on the GMAT

Regards,

Abhijit
_________________

Last edited by Narenn on 20 Mar 2013, 04:56, edited 1 time in total.

Kudos [?]: 3707 [1], given: 2420

Intern
Joined: 01 Feb 2013
Posts: 9

Kudos [?]: 19 [0], given: 9

Location: United States
Concentration: Finance, Technology
GPA: 3
WE: Analyst (Computer Software)
Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

25 Mar 2013, 11:40
My approach:

30/30 -> 1h
30/60 -> 0.5h
---------1.5h total and then 60/1.5 -> 40 (1.5 -> 3 -> 4.5 -> 6)
_________________

Goal: 25 KUDOZ and higher scores for everyone!

Kudos [?]: 19 [0], given: 9

Senior Manager
Joined: 13 May 2013
Posts: 463

Kudos [?]: 197 [1], given: 134

Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

30 Jul 2013, 16:45
1
KUDOS
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

What we have here are equal distances for both segments.

First segment: 30 miles/hour and covered 30 miles, therefore it took one hour.
Second segment: 60 miles/hour and covered 30 miles, therefore it took 1/2 hour.

(Total distance / total time)
(60 / [1hr+ 1/2hr])
(60 / 1.5) = 40 miles avg. speed.

A. 35
B. 40
C. 45
D. 50
E. 55

(B)

When don't we simply add the distances/speeds together to get the average?

Kudos [?]: 197 [1], given: 134

Manager
Joined: 23 Dec 2014
Posts: 50

Kudos [?]: 11 [0], given: 53

Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

03 Feb 2015, 16:58
Rate x Time = Distance
Going: 30 x 1 = 30
Returning: 30 x .5 = 30

Avg speed = Total distance/Total Time
=(30+30)/ (1+.5)
=40

Kudos [?]: 11 [0], given: 53

Intern
Joined: 25 Jan 2016
Posts: 1

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

Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

10 Feb 2016, 21:17
Narenn wrote:
jsphcal wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. If she made no stops during the trip, what was Susan's average speed, in miles per hour, for the entire trip?

a. 35
b. 40
c. 45
d. 50
e. 55

The Common Formula is Average speed = Total distance / Total time.
However if the distances traveled with two different speeds are the same (as 30 in this case), we can calculate the average speed as per following rule.
ratio of two distinct speeds = 1:2
Difference of two distinct speeds = 60 - 30 = 30
Now divide this difference by the sum of intergers of ratio (i.e. by 1+2 = 3 in this case ) 30/3 = 10
Average speed will be 10 X 1 part away from the lower speed i.e. 30 + 10 X 1 = 40.

The benefit of this method is once practiced enough you can calculate the average speed mentally or with minimal calculation and thereby can save valuable time on the GMAT

Regards,

Abhijit

Nice method, but why should it be 10 away from the lower speed and not from the higher speed?

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

Intern
Joined: 18 May 2016
Posts: 20

Kudos [?]: [0], given: 5

Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

20 May 2016, 02:47
1
This post was
BOOKMARKED
Why can't we get her average speed by weighting her speed by the proportion of the total distance realized at a certain speed?

e.g. why is 30(30/60)+60(30/60)=15+30 incorrect?

Thank you please let me know if i haven't been clear enough

Kudos [?]: [0], given: 5

Senior Manager
Joined: 06 Dec 2016
Posts: 256

Kudos [?]: 23 [0], given: 10

Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

24 Feb 2017, 11:02
Hi Bunuel,

I don't know if this approach is right. I got the answer though

distance = speed x time
60 = (30 + 60) x time
60 = 90 x time
60/90 = time
2/3 multiplied by 60 = 40

Therefore, the answer is B. Is my approach correct?

Kudos [?]: 23 [0], given: 10

Math Expert
Joined: 02 Sep 2009
Posts: 41886

Kudos [?]: 128752 [0], given: 12182

Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

25 Feb 2017, 00:08
matthewsmith_89 wrote:
Hi Bunuel,

I don't know if this approach is right. I got the answer though

distance = speed x time
60 = (30 + 60) x time
60 = 90 x time
60/90 = time
2/3 multiplied by 60 = 40

Therefore, the answer is B. Is my approach correct?

No, that;'s not correct.

Distance = Average Speed * Time.

Distance = 60;
Time = 30/30 + 30/60 = 3/2 hours;

60 = Average Speed * 3/2

Average Speed = 40.
_________________

Kudos [?]: 128752 [0], given: 12182

Target Test Prep Representative
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 1634

Kudos [?]: 837 [0], given: 2

Location: United States (CA)
Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

27 Feb 2017, 12:13
anilnandyala wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55

We are given that Susan drove at an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. We must determine her average speed overall. The formula for average speed is:

average speed = total distance/total time

For the first half of the trip, we know that Susan’s speed was 30 mph and her distance was 30 miles, so her time was 30/30 = 1 hour.

For the second half of the trip, we know that Susan’s speed was 60 mph and her distance was 30 miles, so her time was 30/60 = 1/2 hour. Therefore her average speed in miles per hour is:

average speed = (30 + 30)/(1 + ½)

average speed = 60/(3/2)

average speed = 40

_________________

Scott Woodbury-Stewart
Founder and CEO

GMAT Quant Self-Study Course
500+ lessons 3000+ practice problems 800+ HD solutions

Kudos [?]: 837 [0], given: 2

Manager
Joined: 03 Jan 2016
Posts: 59

Kudos [?]: 3 [0], given: 82

Location: India
WE: Engineering (Energy and Utilities)
Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

18 Mar 2017, 20:30
30 miles/ Hour speed for 30 miles => Time Takem = 1 Hr.
60 miles/Hour Speed for remaining 30 miles => Time taken = 0.5 Hour.

Avg Speed = 60/(1+0.5) = 40 mph.

Kudos [?]: 3 [0], given: 82

Intern
Status: Not Applying
Joined: 27 Apr 2009
Posts: 40

Kudos [?]: 315 [0], given: 2

Location: India
Schools: HBS '14 (A)
GMAT 1: 760 Q51 V41
Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

11 May 2017, 05:03
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55

Sol: Average Speed = Total Distance/Total time
Total Distance = 30 miles + 30 miles = 60 miles
Total time = Time taken for the 1st 30 miles + time taken for the next 30 miles
= (30miles/30mph) + (30 miles/60 mph)
= (1 + 0.5) hours
= 1.5 hours

Average speed = Total Distance/Total time = 60/1.5 = 40 mph
_________________

http://www.wiziuscareers.com

Experts in Helping you get through a top-50 school with Low GMAT

Kudos [?]: 315 [0], given: 2

Director
Joined: 13 Mar 2017
Posts: 521

Kudos [?]: 119 [0], given: 62

Location: India
Concentration: General Management, Entrepreneurship
GPA: 3.8
WE: Engineering (Energy and Utilities)
Re: Susan drove an average speed of 30 miles per hour for the [#permalink]

### Show Tags

11 May 2017, 05:12
anilnandyala wrote:
Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55

Time taken in travelling 1st 30 miles = 30/30 = 1 hr
Time taken in travelling remaining 30 miles = 30/60 = 1/2 hr
Total distance traveled = 30+30 = 60 miles
Total time taken = 1+1/2 = 3/2 hr
Average speed = $$\frac{60}{(3/2)}$$ = 40 miles/hr
_________________

MBA Social Network : WebMaggu

Appreciate by Clicking +1 Kudos ( Lets be more generous friends.)

Kudos [?]: 119 [0], given: 62

Re: Susan drove an average speed of 30 miles per hour for the   [#permalink] 11 May 2017, 05:12
Display posts from previous: Sort by