It is currently 17 Oct 2017, 02:30

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

Average Speed Formulas-- Need Help!

Author Message
TAGS:

Hide Tags

Intern
Joined: 12 Feb 2010
Posts: 8

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

Average Speed Formulas-- Need Help! [#permalink]

Show Tags

19 Jan 2011, 06:14
I know that average speed = total distance / total time

But can someone explain another formula I've found on this site:

average speed = (2ab)/(a+b)
a and b represent the average speed of each trip

Where does this come from?

Thanks!

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

Manager
Joined: 19 Sep 2010
Posts: 56

Kudos [?]: 35 [0], given: 9

Location: Pune, India
Re: Average Speed Formulas-- Need Help! [#permalink]

Show Tags

19 Jan 2011, 07:19
suesie970 wrote:
I know that average speed = total distance / total time

But can someone explain another formula I've found on this site:

average speed = (2ab)/(a+b)
a and b represent the average speed of each trip

Where does this come from?

Thanks!

Hey,

This formula is applicable when distances are equal.
example;
Mack travels half of his journey by train at the speed f 120 km/h ad rest half by car at 80 km/h. what is the average speed?
We can solve this problem by many ways.
1. Total distance,take LCM of 120 and 80 =240
therefore,time taken by train=120/120= 1 h
time taken by car = 120/80 = 3/2 h
Average speed = total distance/total time taken
=240/1+3/2
= 96 km/h

2.Method ;
Average speed=2xy/x+y ,where x and y are the speeds
Average speed= 2*120*80/120+80
= 96 km/h

I hope it helps you.

Kudos [?]: 35 [0], given: 9

Intern
Joined: 12 Feb 2010
Posts: 8

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

Re: Average Speed Formulas-- Need Help! [#permalink]

Show Tags

19 Jan 2011, 18:27
SoniaSaini,
Thanks for taking the time to respond. I understand how to use the formulas, I just would like to know where the second one comes from. That is, how was it derived?

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

Intern
Joined: 08 May 2011
Posts: 6

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

GMAT Date: 04-28-2012
Re: Average Speed Formulas-- Need Help! [#permalink]

Show Tags

25 Feb 2012, 08:52
suesie the formula you are referring to - 2ab/(a+b) is derived from Harmonic mean.. As we know that average speed of two speed S1 and S2 is not a AM.. But interestingly it's HM.. So if a vehicle moves with a speed of S1 and S2, covering same distance(you need to be sure of this to apply the HM formula) then you can quickly use the HM formula i.e. 2ab/(a+b)

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

Intern
Joined: 18 Jun 2013
Posts: 1

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

Re: Average Speed Formulas-- Need Help! [#permalink]

Show Tags

18 Jun 2013, 03:24
SoniaSaini wrote:
suesie970 wrote:
I know that average speed = total distance / total time

But can someone explain another formula I've found on this site:

average speed = (2ab)/(a+b)
a and b represent the average speed of each trip

Where does this come from?

Thanks!

Hi Sonia,

Say, I have traveled a distance of 'X' in 'A' kmph and 'Y' in 'B' kmph respectively.

I now have two 'time' values i.e.
X/A---------------(1)
Y/B---------------(2)

Total distance traveled by me = X+Y

Total time taken by me (1)+(2)

i.e. (BX+AY)/(AB)-----------------------------------------*/ I took the LCM /*

Avg Speed = Total distance traveled by me / Total time taken by me

= X+Y / (BX+AY)/(AB)

= (X+Y)AB / (AY+BX)-----------------------------/This is the final formula to find average speed if the distances are different i.e. X<>Y [X is not equal to Y] /

If X=Y

Average speed = (2X)(AB) / (AX+BX) = 2AB/(A+B) ----------------- /Formula of finding average if X=Y/

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

Intern
Joined: 22 Jun 2017
Posts: 1

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

Re: Average Speed Formulas-- Need Help! [#permalink]

Show Tags

22 Jun 2017, 09:49
It's superb I am really confused in average velocity formula yet.but when I came to this website I am cleared my doubt by above formula explanation thank you so much

Posted from my mobile device

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

Re: Average Speed Formulas-- Need Help!   [#permalink] 22 Jun 2017, 09:49
Display posts from previous: Sort by