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Erin travels 42 mile to and from work along the same route.

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Erin travels 42 mile to and from work along the same route. [#permalink] New post 10 Oct 2007, 14:55
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Erin travels 42 mile to and from work along the same route. One morning, she averaged a speed of 45mph going to work and and 67.5 mph coming home. What is the average speed for the round trip?

Pls help me break it down. Thanx.

Last edited by inwosu on 10 Oct 2007, 14:58, edited 1 time in total.
Director
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Re: PS [#permalink] New post 10 Oct 2007, 14:57
inwosu wrote:
Erin travels 42 mile to and from work along the same route. One morning, she averaged a speed of 45mph going to work and and 67.5 miles coming home. What is the average speed for the round trip?

Pls help me break it down. Thanx.


is it supposed to be 67.5mph???
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Re: PS [#permalink] New post 10 Oct 2007, 15:04
inwosu wrote:
Erin travels 42 mile to and from work along the same route. One morning, she averaged a speed of 45mph going to work and and 67.5 mph coming home. What is the average speed for the round trip?

Pls help me break it down. Thanx.


ok this is how i would approach it:

this formula is fundamental => d = rt or r = d/t or t = d/r

1) going to work:
d1 = 42mi
r1 = 45mph
t1 = ?
first find the time:
t1 = d1/r1
= 42/45

2) going from work:
d2 = 42mi
r2 = 67.5mph
t2 = ?
first find the time:
t2 = d2/r2
= 42/67.5

combining the two:
r1+r2 = (d1+d2)/(t1+t2)
= (42+42)/ (42/45 + 42/67.5)
= 84 / (14/15 + 42/67.5)
= 84/ [(14*9) + (42*2)]/135
= 84/ (210/135)
= 54 mph
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same answer [#permalink] New post 10 Oct 2007, 19:14
i also did it using the same way. Is their a shorter way though ?
same answer   [#permalink] 10 Oct 2007, 19:14
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Erin travels 42 mile to and from work along the same route.

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