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January 12th, 2013, 01:28 PM   #11
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Re: Distance, rate, time problem

Quote:
Originally Posted by jc90usa
The problem reads:

a plane flies 600 miles west (into the wind) and makes a return trip following the same flight path. The effect of the jet stream on the plane is 15mph. The round trip takes 9 hours. We want to find the speed of the plane in still air.

In my problem i let the speed of the plane in still air
@v-15..........600...........>t

@v-15..........600...........>9-t

Results in two "different "t's" ...see that?
Agree that's it's confusing: a bit like the missing dollar riddle.
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January 12th, 2013, 02:09 PM   #12
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Re: Distance, rate, time problem

Quote:
@v-15..........600...........>t

@v-15..........600...........>9-t

Results in two "different "t's" ...see that?
Agree that's it's confusing: a bit like the missing dollar riddle.
Thanks for letting me know about the missing dollar riddle, never heard of it.

So why do i need to plugin (v+15) into t, instead of (v-15)?
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January 12th, 2013, 08:01 PM   #13
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Re: Distance, rate, time problem

Quote:
Originally Posted by jc90usa
I ask because I tried to make


Once I get them to equal, I substituted t using t=d/r
I am not sure why when I do this method I need to use


If I use,


I need to use to get the correct answer. Why does interchanging t for (9-t) affect it?
Well, my final attempt at trying to explain:
(1): the "t" here is the result of travelling 600 miles at speed v+15 ; t = 600 / 150 = 4
(2): the "t" here is the result of travelling 600 miles at speed v-15 ; t = 600 / 120 = 5

So the t in (1) is different from the t in (2). Good luck.
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January 12th, 2013, 11:26 PM   #14
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Re: Distance, rate, time problem

I got it!!! Thank you Denis and Mark, for your patience and explanations!

After all your feedback and my working out problems in different ways, I understood what I was doing wrong.

For anyone else having problems with these because they are asked to use a pesky table to represent d=rt,
just remember that you are simply dealing with two rows of equations.

First try to see if anything in the word problem points to "adding" the distances or the time. If it does, try using an addition equation. If anything in the word problem is equal, then you can try setting up an equality.

IF you have to make substitutions, don't use the general d=rt and then reason what is required. (could work but prone to error, or ambiguity)
USE either d=rt you created from the word problem to solve for the variable you need!

The confusion is that the "addition" or "equality" method requires you to use d=rt generally to setup your addition or equality,
but when substitution is required, a filled in d=rt is required.

Being comfortable with those two, you should be able to handle if two unknowns are given.

Happy day
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January 13th, 2013, 04:38 AM   #15
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Re: Distance, rate, time problem

Hey you can apply for your pilot's liscense now
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January 13th, 2013, 11:07 AM   #16
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Re: Distance, rate, time problem

hmmm m. that is an idea, lol
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