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sohaib_iba: Junior | Next Rank: 30 Posts; Posts: 12; Joined: Wed Feb 24, 2010 7:09 am

by sohaib_iba » Wed Jun 02, 2010 7:24 am

Ahsan travels from A to B at a speed of 40 mph and returns along the same route a speed of 80 mph. what is his average speed for the round trip (in mph)?

A) 46.33
B) 48
C) 53.33
D) 60
E) 65.33

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Source: Beat The GMAT — Problem Solving | Wed Jun 02, 2010 7:24 am

selango: Legendary Member; Posts: 1460; Joined: Tue Dec 29, 2009 1:28 amFollowed by: 7 members; Thanked: 135 times

by selango » Wed Jun 02, 2010 7:30 am

Average speed =2*40*80/(40+80)

=53.33

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pradeepkaushal9518: Legendary Member; Posts: 1309; Joined: Wed Mar 17, 2010 11:41 pmFollowed by: 5 members; Thanked: 33 times

by pradeepkaushal9518 » Wed Jun 02, 2010 7:33 am

ANSWERT IS C

SUPPOSE distance is x

so total distance=2x
speed =distance /time

40=x/t1
t1=x/40

60=x/t2
t2=x/60

speed =total distance/total time
=2x/(x/40+x/80)

soving we get 53.33

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selango: Legendary Member; Posts: 1460; Joined: Tue Dec 29, 2009 1:28 amFollowed by: 7 members; Thanked: 135 times

by selango » Wed Jun 02, 2010 7:37 am

There are 2 formulas related to Average Speed

1.If object travels from point A to point b with speed p and point B to point A with speed q.

Average speed=2pq/(p+q)

2.If object covers part of journey at speed p and remaning at speed q and diatance are in ration m:n

Average speed=(m+n)pq/(mq+np)

Last edited by selango on Wed Jun 02, 2010 8:23 am, edited 1 time in total.

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liferocks: Legendary Member; Posts: 576; Joined: Sat Mar 13, 2010 8:31 pmFollowed by: 1 member; Thanked: 97 times

by liferocks » Wed Jun 02, 2010 7:38 am

since ratio of speed is 1:2
ratio of time taken is 2:1
so let the time taken while going be 2t and coming be t
so distance is 40*2t*2=80t*2=160t
time taken =3t
so avarage speed=160t/3t =53.33..option C

"If you don't know where you are going, any road will get you there."
Lewis Carroll

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indiantiger: Master | Next Rank: 500 Posts; Posts: 362; Joined: Fri Oct 02, 2009 4:18 amFollowed by: 1 member; Thanked: 26 times

by indiantiger » Wed Jun 02, 2010 8:04 am

total distance traveled is 2d

total time taken = d/40 + d/80 (as time = distance/speed) = d (120/3200) = 3d/80

avg speed = total distance / total time
= 2d / (3d/80) - 160/3 = 53.33 (answer)

"Single Malt is better than Blended"

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