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# Algebra Equation -

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Intern
Joined: 04 Nov 2012
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Schools: HBS '16, Wharton '16

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18 Jun 2013, 18:32
Hi,

Question about writing equations for rate/distance problems.

If the problem states, train A starts traveling at 40 mph, and train B leaves 3 hours later at 60 mph. After how long will they catch up.

One way, correct, to write it would be : 40 (t + 3) = 60 t ---- since the distance traveled will be the same.

BUT

why can i not, and the answer is different, if i write it like this : 60 (t - 3) = 40 t ...

I am having a hard time understanding this ..... if someone could please elaborate

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Veritas Prep GMAT Instructor
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18 Jun 2013, 20:31
indiheats wrote:
Hi,

Question about writing equations for rate/distance problems.

If the problem states, train A starts traveling at 40 mph, and train B leaves 3 hours later at 60 mph. After how long will they catch up.

One way, correct, to write it would be : 40 (t + 3) = 60 t ---- since the distance traveled will be the same.

BUT

why can i not, and the answer is different, if i write it like this : 60 (t - 3) = 40 t ...

I am having a hard time understanding this ..... if someone could please elaborate

You can use either way. Note that the variable t stands for different things in the two equations.

40 (t + 3) = 60 t (t is the time for which train B travels)
t = 6 hrs
Train B travels for 6 hrs
Train A travels for 9 hrs

60 (t - 3) = 40 t (t is the time for which train A travels)
t = 9
Train A travels for 9 hrs
Train B travels for 9 - 3 = 6 hrs
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Re: Algebra Equation -   [#permalink] 18 Jun 2013, 20:31
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