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Math Help - Simple Math Question Help

  1. #1
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    Question Simple Math Question Help

    Hi. I have a question about a "famous" type of word problems. I always come across problems like "how many integers below 2000 are divisible by 3 and 5" or "how many odd numbers below 5000 are not divisible by 9." The only way I know how to do these problems is by trying to find a pattern by writing everything out, which takes a lot of time. Is there any easy way to do these quickly? Thanks!
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  2. #2
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    Quote Originally Posted by sleigh View Post
    "how many integers below 2000 are divisible by 3 and 5"
    \left\lfloor {\frac{{1999}}<br />
{3}} \right\rfloor  + \left\lfloor {\frac{{1999}}<br />
{5}} \right\rfloor  - \left\lfloor {\frac{{1999}}<br />
{{15}}} \right\rfloor
    WHY?
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