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Math Help - Basic Probability

  1. #1
    Senior Member Stroodle's Avatar
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    Basic Probability

    A whole number between 1 and 300 (inclusive) is chosen at random. Find the probability that the number is:

    (a) divisible by 3 (b) divisible by 4 (c) divisible by 3 or by 4

    So I know that (a) =\frac{1}{3} and (b) =\frac{1}{4}, but I was wondering if there's a quick method to find the solution for (c)

    Thanks for your help.
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  2. #2
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    Quote Originally Posted by Stroodle View Post
    A whole number between 1 and 300 (inclusive) is chosen at random. Find the probability that the number is:
    (a) divisible by 3 (b) divisible by 4 (c) divisible by 3 or by 4
    So I know that (a) =\frac{1}{3} and (b) =\frac{1}{4}, but I was wondering if there's a quick method to find the solution for (c)
    Well there are 25 numbers in that range divisible by 3 & 4.
    P(3\cup 4)=P(3)+P(4)-P(3\cap 4)
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  3. #3
    Senior Member Stroodle's Avatar
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    Quote Originally Posted by Plato View Post
    Well there are 25 numbers in that range divisible by 3 & 4.
    P(3\cup 4)=P(3)+P(4)-P(3\cap 4)
    Yes, but is there a some kind of method of finding the 25 numbers, without going through all of the even numbers from 0-300?
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  4. #4
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    Quote Originally Posted by Stroodle View Post
    Quote Originally Posted by Plato View Post
    Well there are 25 numbers in that range divisible by 3 & 4.
    P(3\cup 4)=P(3)+P(4)-P(3\cap 4)
    Yes, but is there a some kind of method of finding the 25 numbers, without going through all of the even numbers from 0-300?
    If a number is divisible by 3 and divisible by 4, then it has to be divisible by 12. So how many numbers from 1 to 300 are divisible by 12?


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  5. #5
    Senior Member Stroodle's Avatar
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    Ahh. Of course. THanks for that!
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