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Math Help - need help

  1. #1
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    need help

    There are 3 cars going on a autobahn. At the end of the autobahn there are 3 turnpikes. What is the probability that one of them will pass through a different turnpike than the others and the other 2 will pass through the same turnpike.
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  2. #2
    Junior Member Barioth's Avatar
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    Re: need help

    The first car has 3 turnpike he can take, the second car as 2 way or choosing a turnpike and the third one must take the same as the second one, so he only has 1 choice.

    You end up with 3*2*1=3!=6.
    Now since we've probability, we'll guess each car as the same %chance of going in each turnpike, so we must count how many possible way there is.

    Every car as 3 choices so you end up with 3*3*3=27

    So the probabily is 6/27=2/9

    That look good to me. But I might be wrong, so be sure to double check
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  3. #3
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    Re: need help

    Hello, kastamonu!

    There are 3 cars going on a autobahn. At the end of the autobahn there are 3 turnpikes.
    What is the probability that one of them will pass through a different turnpike than the others
    and the other two will pass through the same turnpike?

    Each car has a choice of 3 turnpikes.
    . . There are 3^3 = 27 possible outcomes.

    Suppose all three use the same turnpike.
    . . There are 3 ways: AAA,\,BBB,\,CCC

    Suppose all three use different turnpikes.
    . . There are 3! = 6 ways: \{ABC,\,AC\!B,\,B\!AC,\,BC\!A,\,C\!AB,\,C\!BA\}

    Hence, there are: . 27 - 3 - 6 \:=\:18 ways.

    \text{Probability} \:=\:\frac{18}{27} \:=\:\frac{2}{3}
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  4. #4
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    Re: need help

    Many Thanks.
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  5. #5
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    Re: need help

    Many Thanks. But first solution seems more reasonable.
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  6. #6
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    Re: need help

    Sorry, but Soroban's answer is the correct one. Barioth's answer is the probability the a specified one of the cars, specifically, the one he calls "the first car", takes one of the turnpike and the other two both take the same one of the other two. Soroban's answer is the probability that "one car takes one turn pike and the other two take the same one of the other two" which was the question.

    Because there are 3 cars, and any one can be the car that goes "alone", so you can get the correct answer by multiplying Barioth's answer by 3: 3(2/9)= 2/3.
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  7. #7
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    Re: need help

    I am sorry Soroban.

    Many Thanks HallsofIvy for warning me.
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  8. #8
    Junior Member Barioth's Avatar
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    Re: need help

    Quote Originally Posted by HallsofIvy View Post
    Sorry, but Soroban's answer is the correct one. Barioth's answer is the probability the a specified one of the cars, specifically, the one he calls "the first car", takes one of the turnpike and the other two both take the same one of the other two. Soroban's answer is the probability that "one car takes one turn pike and the other two take the same one of the other two" which was the question.

    Because there are 3 cars, and any one can be the car that goes "alone", so you can get the correct answer by multiplying Barioth's answer by 3: 3(2/9)= 2/3.
    That's right, flame on me. Sorry!
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