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# Lhotse-face bikes to work in the morning at a constant speed

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Lhotse-face bikes to work in the morning at a constant speed [#permalink]  02 Apr 2006, 23:05
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Lhotse-face bikes to work in the morning at a constant speed of 4km/hr ( ignores the red lights ) and returns home by the same route. What is Lhotse-face's average speed for the entire trip ?

A. The constant average speed on the return journey is 6km/hr.
B. Total distance traveled is 24 kms.
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Total distance/total time traveled

24/(12/4+12/6)=average speed
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Hmm.. I would say it is A, assuming that Lhotse-face drove on the same track both directions..

Stm 1:
If x is the distance travelled in each direction
Avg speed = 2x/(t1+t2)

t1 = x/4 & t2 = x/6

Avg speed = 2x / (x/4+x/6) = 4.8 SUFF

Stm2:
We don't know the speed of return journey.
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....

giddi77 wrote:
Hmm.. I would say it is A, assuming that Lhotse-face drove on the same track both directions..

Stm 1:
If x is the distance travelled in each direction
Avg speed = 2x/(t1+t2)

t1 = x/4 & t2 = x/6

Avg speed = 2x / (x/4+x/6) = 4.8 SUFF

Stm2:
We don't know the speed of return journey.
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A seems plausible according to giddi
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1) distance to work = x
time taken to get to work = x/4
time taken to get hom = x/6

we can calculate total time, and then use this to get the average speed. the 'x' will cancel out.

Sufficient.

2) We do not have the speed or time taken for the return trip. Insufficient.

Ans A
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If you know thw speeds in both directions say x and y mph

Av speed = 2ab/(a+b)

So its A.
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Are we to assume that he goes back the same way? This is when i get confused abt these questions.
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I would say A as well. Regardless if you plug in different total distances (e.g. 24 or 48) you will get the same avg. speed (i.e. 4.8).
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I wasn't saying a different distance but I was thinking the same rate per hour.. asumming that he passed all the red lights etc. on his way back... should we assume that...
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AshikaP wrote:
I wasn't saying a different distance but I was thinking the same rate per hour.. asumming that he passed all the red lights etc. on his way back... should we assume that...

You can't assume the same speed for both the journey to work and from work. The only thing constant here is the distance.
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