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Math Help - integers

  1. #1
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    integers

    how many positive integers can be expressed as a product of two or more of the prime numbers 5,7,11 and 13 if no one of the product should include the same prime factor more than once...

    actually i multiplied 5*7*11*13 and got 5005 so i thought the answer will be like all multiples of 5005 but the answer was 11 .so can any one tell me the solution for this question
    Last edited by CaptainBlack; August 16th 2009 at 11:02 AM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by bluffmaster.roy.007 View Post
    how many positive integers can be expressed as a product of two or more of the prime numbers 5,7,11 and 13 if no one of the product should include the same prime factor more than once...

    actually i multiplied 5*7*11*13 and got 5005 so i thought the answer will be like all multiples of 5005 but the answer was 11 .so can any one tell me the solution for this question
    Well we start with the products of exactly two of the primes: 5.7, 5.11, 6.13, 7.11, 7.13, 11.13.

    Then the products of three: 5.7.11, 5.7.13, 5.11.13, 7.11.13

    Finally there is one product of all four: 5.7.11.13

    So how many numbers are there?

    CB
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  3. #3
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    thanks man
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