• 7 CATs FREE!
    If you earn 100 Forum Points

    Engage in the Beat The GMAT forums to earn
    100 points for $49 worth of Veritas practice GMATs FREE

    Veritas Prep
    VERITAS PRACTICE GMAT EXAMS
    Earn 10 Points Per Post
    Earn 10 Points Per Thanks
    Earn 10 Points Per Upvote
    REDEEM NOW

Two methods to calculate total average speed

This topic has expert replies
Senior | Next Rank: 100 Posts
Posts: 50
Joined: 04 Oct 2007
Location: RTP, NC
Thanked: 26 times
Followed by:3 members
In problems that ask you to calculate total average speed, given the speed for onward and return trip, you could apply a formula or a neat trick to get to the answer faster. Lets derive the genral formula first and see how it applies it to couple of examples.

Say, a car travels at S1 mph on a trip and at S2 mph on return trip. What is its average speed for the entire trip?
Solution:
*** Don't fall in the trap of just averaging the 2 speeds. Overall average speed is not (S1+S2)/2. ***
Total average speed is simply = Total distance/Total time
Lets say,
D = distance travelled by the car in EACH direction
t1 = time spent on onward trip
t2 = time spent on return trip
Thus, the total distance travelled by the car = D+D= 2D
And, by the formula, Speed = Distance/Time
S1 = D/t1 => t1 = D/S1
S2 = D/t2 => t2 = D/S2
Total average speed = Total Distance/Total time = 2D/(t1+t2) = 2D/(D/S1+D/S2) = 2S1*S2/(S1+S2)

Remember this general formula for a total average speed problems:
Total average speed = 2S1*S2/(S1+S2)

Example:
A car travels at 60 mph on a trip and at 100 mph on return trip. What was its average speed for the entire trip?
Solution:
*** Total average speed is not (60+100)/2 = 80 ***
Total average speed = 2*60*100/(100+60) = 2*60*100/160 = 2*60*5/8 = 60*5/4 = 15*5 = 75

Alternatively, you may want to check if the following trick saves you some time.
Calculate the ratio of the speeds r1:r2. In our example it is 60:100 = 3:5
Then divide the difference between the speeds (s2-s1) by r1+r2 to get one part. In our example (100-60)/(3+5) = 5 is one part
The required answer is r1 parts away from the lower speed. That is, 60+r1*5 = 60+3*5 = 75 mph

Lets check how it works for S1=20 mph and S2=40 mph
Method 1:
Using the formula Total avg speed = 2S1*S2/(s1+s2)
= 2*20*40/(20+40)
= 2*20*40/60
= 80/3 = 26.67 mph
Method 2:
Ratio r1:r2 = 20:40 = 1:2
r1+r2 = 3
1 part = (S2-S1)/(r1+r2) = (40-20)/3 = 20/3 = 6.67
Total Avg speed is r1 parts away from smaller speed
Therefore avg speed = 20+ r1*6.67 = 20+1*6.67 = 26.67 mph


Hope you find this useful.
-Ashish

GMAT/MBA Expert

User avatar
Site Admin
Posts: 6728
Joined: 13 Feb 2006
Location: Los Angeles, CA
Thanked: 1249 times
Followed by:980 members

by beatthegmat » Tue Nov 13, 2007 10:39 pm
This is really great Ashish! I'm adding this to the Beat The GMAT del.icio.us.
Beat The GMAT | The MBA Social Network
Community Management Team

Research Top GMAT Prep Courses:
https://www.beatthegmat.com/gmat-prep-courses

Research The World's Top MBA Programs:
https://www.beatthegmat.com/mba/school

Newbie | Next Rank: 10 Posts
Posts: 1
Joined: 06 Jan 2008
Can you explain how this works? 2D/(D/S1+D/S2) = 2S1*S2/(S1+S2)

Thank you

Newbie | Next Rank: 10 Posts
Posts: 1
Joined: 27 Jan 2008
Location: North Vancouver, BC, Canada
Thanked: 1 times
butler wrote:Can you explain how this works? 2D/(D/S1+D/S2) = 2S1*S2/(S1+S2)
To simplify the fractions in the formula, you need to multiply the numerator and denominator by the same value (remember that multiplying by X/X = 1 retains the value of the expression). In the case of the formula above, we need to multiply top and bottom by S1*S2 (common denominator for the fractions D/S1 and D/S2).
So here is the derivation, not leaving out any steps...

2 * D / (D/S1 + D/S2)
Multiply top and bottom by S1 * S2
= 2 * D * S1 * S2 / ((D/S1 + D/S2) * S1 * S2)
Distribute the multipliers in the denominator
= 2 * D * S1 * S2 / (D * S2 + D * S1)
Factor out the D from the denominator (distributive law)
= 2 * D * S1 * S2 / D (S2 + S1)
Cancel the D factor from top and bottom
= 2 * S1 * S2 / (S2 + S1)
Addition is of course commutative, so the order of the terms in the denominator can be reversed ...
= 2 * S1 * S2 / (S1 + S2)

Of course it saves time to just memorize the formula so that you don't have to derive it unless you forget. However, recognize the danger of applying the formula to a distance/speed/time question that doesn't exactly match this pattern (for what is asked).

I hope this helps. If you need more practice on simplification and you happen to have the Princeton Review "Cracking the GMAT 2008 edition", I suggest you go through the Fractions section on pp. 77-81. Remember that in algebra, the rules for arithimetic operations (associative law, commutative law, distributive law, etc) are the same with variables as they are for number constants.

Newbie | Next Rank: 10 Posts
Posts: 8
Joined: 14 Sep 2007
Thanked: 1 times
GMAT Score:650

One point

by landroni » Thu Oct 16, 2008 1:04 am
I wanted to stress one point (idea taken from this Math Review).

Since we're on to averaging speeds, the speeds will be different in all the cases.
achandwa wrote:Say, a car travels at S1 mph on a trip and at S2 mph on return trip. What is its average speed for the entire trip?

Remember this general formula for a total average speed problems:
Total average speed = 2S1*S2/(S1+S2)
This is the harmonic mean. This formula applies when the situation is:
- same distance (onward and return trip)

As long as you're in a `same or equal distances' situation, any information on time is pretty much irrelevant.
achandwa wrote:*** Don't fall in the trap of just averaging the 2 speeds. Overall average speed is not (S1+S2)/2. ***
You would use the weighted arithmetic mean when the situation is:
- different distance
- different time

The `time' is the frequency, or weight factor. You would use the simple arithmetic mean in a simplified situation:
- different distance
- same time

The `time' weight factor has no impact on the average formula, since it factors itself out.

Liviu

Master | Next Rank: 500 Posts
Posts: 140
Joined: 28 Feb 2009
Location: India
Thanked: 14 times
Followed by:3 members
A man walks by speed 2 KM/hr and cover 3/4 of total distance in 10 minutes and reaming distance in 30 minutes. What is the total distance ?

A. 400
B. 4.80
C. 360
D. 40
E. None
Regards,
Farooq Farooqui.
London. UK

It is your Attitude, not your Aptitude, that determines your Altitude.

Master | Next Rank: 500 Posts
Posts: 177
Joined: 14 Aug 2008
Thanked: 25 times

by mp2437 » Wed Oct 28, 2009 6:53 am
I say choice E. I am getting an answer of 4/9 km.

User avatar
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: 01 Mar 2010

by hkyoon » Sun Mar 28, 2010 5:07 pm
Yes, I do too

2km/hr * 1/6 hr = 3/4 * Total Distance

User avatar
Master | Next Rank: 500 Posts
Posts: 435
Joined: 15 Mar 2010
Thanked: 32 times
Followed by:1 members
Well its obviously E. How can a man take longer for 1/4th the distance than 3/4th of it

And great contribution Ashish. That sure was informative.
Just as an add on, I have attached for 3 speeds or greater too. Check it out
Image

Newbie | Next Rank: 10 Posts
Posts: 1
Joined: 29 Jun 2010

by dhruv pillai » Tue Jun 29, 2010 8:29 am
achandwa wrote:In problems that ask you to calculate total average speed, given the speed for onward and return trip, you could apply a formula or a neat trick to get to the answer faster. Lets derive the genral formula first and see how it applies it to couple of examples.

Say, a car travels at S1 mph on a trip and at S2 mph on return trip. What is its average speed for the entire trip?
Solution:
*** Don't fall in the trap of just averaging the 2 speeds. Overall average speed is not (S1+S2)/2. ***
Total average speed is simply = Total distance/Total time
Lets say,
D = distance travelled by the car in EACH direction
t1 = time spent on onward trip
t2 = time spent on return trip
Thus, the total distance travelled by the car = D+D= 2D
And, by the formula, Speed = Distance/Time
S1 = D/t1 => t1 = D/S1
S2 = D/t2 => t2 = D/S2
Total average speed = Total Distance/Total time = 2D/(t1+t2) = 2D/(D/S1+D/S2) = 2S1*S2/(S1+S2)

Remember this general formula for a total average speed problems:
Total average speed = 2S1*S2/(S1+S2)

Example:
A car travels at 60 mph on a trip and at 100 mph on return trip. What was its average speed for the entire trip?
Solution:
*** Total average speed is not (60+100)/2 = 80 ***
Total average speed = 2*60*100/(100+60) = 2*60*100/160 = 2*60*5/8 = 60*5/4 = 15*5 = 75

Alternatively, you may want to check if the following trick saves you some time.
Calculate the ratio of the speeds r1:r2. In our example it is 60:100 = 3:5
Then divide the difference between the speeds (s2-s1) by r1+r2 to get one part. In our example (100-60)/(3+5) = 5 is one part
The required answer is r1 parts away from the lower speed. That is, 60+r1*5 = 60+3*5 = 75 mph

Lets check how it works for S1=20 mph and S2=40 mph
Method 1:
Using the formula Total avg speed = 2S1*S2/(s1+s2)
= 2*20*40/(20+40)
= 2*20*40/60
= 80/3 = 26.67 mph
Method 2:
Ratio r1:r2 = 20:40 = 1:2
r1+r2 = 3
1 part = (S2-S1)/(r1+r2) = (40-20)/3 = 20/3 = 6.67
Total Avg speed is r1 parts away from smaller speed
Therefore avg speed = 20+ r1*6.67 = 20+1*6.67 = 26.67 mph


Hope you find this useful.
-Ashish
thank you Mr Ashish

Newbie | Next Rank: 10 Posts
Posts: 1
Joined: 17 Nov 2010

by peertje888 » Wed Nov 17, 2010 1:17 am
To see if it made any difference calculating the average wind speed of a turbine I made a test in Excel. In this scenario the duration for every bin is the same. That is why it didn't make any difference.
Attachments
TotalAverage.xlsx
(9.24 KiB) Downloaded 100 times

User avatar
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: 29 Apr 2011

by Uvin » Fri Apr 29, 2011 10:14 am
a object travels on a strait line has V1 velocity for half of its journey. For the rest, it gets V2 velocity for the first half of the rest, and then V2 velocity for the other half. What is the average speed of the object ?
Can anybody help me with this question???
thank you!

Junior | Next Rank: 30 Posts
Posts: 13
Joined: 28 Apr 2011
Thanked: 1 times

by tapanu » Wed May 04, 2011 5:15 am
Thanks Ashish !!!
Very helpful !!
The will to win is worthless if you do not have the will to prepare.

Newbie | Next Rank: 10 Posts
Posts: 2
Joined: 03 Jul 2011
Thanked: 1 times

by ajaygiri » Tue Jul 12, 2011 2:22 pm
thank you ashish!

could not follow the ratio logic though, but the first one is crystal. thanks for the lucid explanation.

Senior | Next Rank: 100 Posts
Posts: 82
Joined: 04 Nov 2011
Followed by:2 members

by amit28it » Sat Dec 24, 2011 3:43 am
achandwa wrote:In problems that ask you to calculate total average speed, given the speed for onward and return trip, you could apply a formula or a neat trick to get to the answer faster. Lets derive the genral formula first and see how it applies it to couple of examples.

Say, a car travels at S1 mph on a trip and at S2 mph on return trip. What is its average speed for the entire trip?
Solution:
*** Don't fall in the trap of just averaging the 2 speeds. Overall average speed is not (S1+S2)/2. ***
Total average speed is simply = Total distance/Total time
Lets say,
D = distance travelled by the car in EACH direction
t1 = time spent on onward trip
t2 = time spent on return trip
Thus, the total distance travelled by the car = D+D= 2D
And, by the formula, Speed = Distance/Time
S1 = D/t1 => t1 = D/S1
S2 = D/t2 => t2 = D/S2
Total average speed = Total Distance/Total time = 2D/(t1+t2) = 2D/(D/S1+D/S2) = 2S1*S2/(S1+S2)

Remember this general formula for a total average speed problems:
Total average speed = 2S1*S2/(S1+S2)

Example:
A car travels at 60 mph on a trip and at 100 mph on return trip. What was its average speed for the entire trip?
Solution:
*** Total average speed is not (60+100)/2 = 80 ***
Total average speed = 2*60*100/(100+60) = 2*60*100/160 = 2*60*5/8 = 60*5/4 = 15*5 = 75

Alternatively, you may want to check if the following trick saves you some time.
Calculate the ratio of the speeds r1:r2. In our example it is 60:100 = 3:5
Then divide the difference between the speeds (s2-s1) by r1+r2 to get one part. In our example (100-60)/(3+5) = 5 is one part
The required answer is r1 parts away from the lower speed. That is, 60+r1*5 = 60+3*5 = 75 mph

Lets check how it works for S1=20 mph and S2=40 mph
Method 1:
Using the formula Total avg speed = 2S1*S2/(s1+s2)
= 2*20*40/(20+40)
= 2*20*40/60
= 80/3 = 26.67 mph
Method 2:
Ratio r1:r2 = 20:40 = 1:2
r1+r2 = 3
1 part = (S2-S1)/(r1+r2) = (40-20)/3 = 20/3 = 6.67
Total Avg speed is r1 parts away from smaller speed
Therefore avg speed = 20+ r1*6.67 = 20+1*6.67 = 26.67 mph


Hope you find this useful.
-Ashish
Yes Ashish it was really helpful .I was also having the same doubt in this question .It was not exactly same as it is .But in my question there is same logic as in this question .So i was able to solve my question and is able to know how to simplify ratios .Thanks for your support .