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Homework Help: Quick algebra based question

  1. Sep 6, 2006 #1
    Train leaves station going 50MPH
    Car leaves station going 70MPH - 2 hours later

    How far til car catches train?

    This should be so simple, here is what I've done:

    d=50t
    d=70(t-2)

    50t=70(t-2)
    50t=70t-140
    140=20t

    I get t=7 miles for my final answer, but according to the book this is wrong. Why?
     
  2. jcsd
  3. Sep 6, 2006 #2
    try it this way: when the car starts the train is 100 miles away. the car makes up ground at 20 miles/hour (70-50)... how many hours does it take to catch the train... how many miles did the car go in that time.
     
  4. Sep 6, 2006 #3
    How did you get miles from that? If
    d=50mph*t hours
    d=70mph*(t-2)hours

    50mph*t hours=70mph*(t-2)hours
    50mph*t hours=70mph*t hours-140 miles
    140 miles=20mph*t hours
    140miles/20mph = t hours
    7 hours = t hours
     
  5. Sep 6, 2006 #4

    Kurdt

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    Science Advisor
    Gold Member

    I think the problem is that you've solved for t when the question asks you to solve for d, the distance.

    Of course you can now just put t back into one of the equations.
     
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