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December 11th, 2007, 01:46 PM   #1
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help on distance, rate, time

i have this problem for math, and i can't figure out how to get started on setting it up.

jenny travels 90 kph to her cousin's house and returns the next day at 60 kph. if the whole trip took 7 hours, how far is it from jenny's house to her cousins house?
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December 11th, 2007, 06:24 PM   #2
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Well, this isn't such a hard problem.
The distance from both houses will be the same going there as coming back. The time she takes to get there will be faster than the time she takes to get back.

Since the whole trip takes 7 hours, we can use 7-t for the time she took to get back home.

And it's just simple d=rt

d=90t
d=60(7-t)

Set d equal to each other:

90t=60(7-t)

Solve for t, and when you get the answer, plug it into one of the first two equations to get the distance from her house to her cousin's house.
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December 11th, 2007, 08:13 PM   #3
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As above,
d=90t * * * * * [1]
d=60(7-t) * * [2]

Divide [1] by 90 and [2] by 60 and add the resulting equations to obtain
d/90 + d/60 = 7,
so d = 7/(1/90 + 1/60)
* * * * = 252.
That suggests just memorizing a simple formula for this type of problem. Compare to the problem of determining what constant voltage to apply across two resistors (90Ω and 60Ω) in parallel so that the total currrent through them is 7 amps.
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