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need helpppppp

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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by selango » Wed Jun 02, 2010 7:30 am
Average speed =2*40*80/(40+80)

=53.33
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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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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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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."
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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)
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