GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 20 Oct 2018, 08:53

### 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

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

Author Message
TAGS:

### Hide Tags

Manager
Joined: 07 Feb 2010
Posts: 130
Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

Updated on: 13 Oct 2013, 03:01
8
20
00:00

Difficulty:

5% (low)

Question Stats:

79% (00:36) correct 21% (00:37) wrong based on 1160 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

Originally posted by anilnandyala on 02 Nov 2010, 06:56.
Last edited by Bunuel on 13 Oct 2013, 03:01, edited 1 time in total.
Moved to PS forum.
Math Expert
Joined: 02 Sep 2009
Posts: 50007
Re: susan drove an average speed  [#permalink]

### Show Tags

02 Nov 2010, 07:04
7
7
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;

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

_________________
##### General Discussion
Senior Manager
Joined: 24 Aug 2009
Posts: 475
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

Manager
Status: Looking to improve
Joined: 15 Jan 2013
Posts: 156
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

MBA Section Director
Status: Back to work...
Affiliations: GMAT Club
Joined: 22 Feb 2012
Posts: 5682
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

Updated on: 20 Mar 2013, 04:56
2
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
_________________

Originally posted by Narenn on 20 Mar 2013, 02:09.
Last edited by Narenn on 20 Mar 2013, 04:56, edited 1 time in total.
Intern
Joined: 01 Feb 2013
Posts: 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
2
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!

Senior Manager
Joined: 13 May 2013
Posts: 429
Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

30 Jul 2013, 16:45
1
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?
Intern
Joined: 23 Dec 2014
Posts: 48
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
Intern
Joined: 25 Jan 2016
Posts: 1
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?
Intern
Joined: 18 May 2016
Posts: 29
Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

20 May 2016, 02:47
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
Senior Manager
Joined: 06 Dec 2016
Posts: 250
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?
Math Expert
Joined: 02 Sep 2009
Posts: 50007
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.
_________________
Target Test Prep Representative
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 3896
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

Manager
Joined: 03 Jan 2016
Posts: 59
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.
Intern
Status: Not Applying
Joined: 27 Apr 2009
Posts: 47
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

Director
Joined: 13 Mar 2017
Posts: 622
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
_________________

CAT 2017 99th percentiler : VA 97.27 | DI-LR 96.84 | QA 98.04 | OA 98.95
UPSC Aspirants : Get my app UPSC Important News Reader from Play store.

MBA Social Network : WebMaggu

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

What I believe is : "Nothing is Impossible, Even Impossible says I'm Possible" : "Stay Hungry, Stay Foolish".

Intern
Joined: 14 May 2016
Posts: 23
Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

21 Jan 2018, 11:10
Here is a very easy formula to remember:

Let a be the first speed
Let b be the second speed

Avg Speed = 2ab/a+b

a = 30
b= 60

Avg Speed = 2(30)(60)/30+60 = 3600/90 = 40
EMPOWERgmat Instructor
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 12685
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

26 Feb 2018, 19:39
Hi All,

To figure out Average Speed for an entire trip, we need TOTAL Distance and TOTAL Time:

Total Distance = (Average Speed)(Total Time)

Here, we're given all of the numerical information we need to calculate each part of the journey:

1st part: 30 miles/hour for 30 miles

30 miles = (30mph)(Time)
30/30 = T hours
T = 1 hour

2nd part: 60 miles/hour for 30 miles
30 miles = (60mph)(Time)
30/60 = T hours
T = 1/2 hour

Total Distance = 30 + 30 = 60 miles
Total Time = 1 + 1/2 = 1.5 hours

Using these values, we can plug in and find the Average Speed for the entire trip:

60 miles = (Average Speed)(1.5 hours)
60/1.5 = Average Speed

Average Speed = 40 mph

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

760+: Learn What GMAT Assassins Do to Score at the Highest Levels
Contact Rich at: Rich.C@empowergmat.com

# Rich Cohen

Co-Founder & GMAT Assassin

Special Offer: Save \$75 + GMAT Club Tests Free
Official GMAT Exam Packs + 70 Pt. Improvement Guarantee
www.empowergmat.com/

*****Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*****

Intern
Joined: 08 Jul 2017
Posts: 26
GMAT 1: 620 Q48 V29
Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

26 Feb 2018, 22:33
Bunuel wrote:
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;

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

but i think av should be closer to the 60
Math Expert
Joined: 02 Sep 2009
Posts: 50007
Re: Susan drove an average speed of 30 miles per hour for the  [#permalink]

### Show Tags

26 Feb 2018, 23:14
robu1 wrote:
Bunuel wrote:
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;

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

but i think av should be closer to the 60

It's not clear what you are trying to say. The average speed is 40 miles per hour as shown in all solutions above. Do you have any questions?
_________________
Re: Susan drove an average speed of 30 miles per hour for the &nbs [#permalink] 26 Feb 2018, 23:14

Go to page    1   2    Next  [ 25 posts ]

Display posts from previous: Sort by