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Math Help - I need help with a calculation thats baffling me! (easily done!)

  1. #1
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    I need help with a calculation thats baffling me! (easily done!)

    Hi all,

    i'm not a mathematician therefore I need help! is there a calculation that determines how many times a smaller number has to be doubled to achieve a larger number? also will it work where doubling a number doesn't exactly fit into the larger number?

    for example I know that 10 doubled three times equals 80 but is there a calculation where we know of 10 & 80 but it calculates that 10 needs to be calculated 3 times?

    Cheers for any help!

    Morgan
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  2. #2
    Senior Member
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    Use logarithms.
    If you need to find x in:
    2^x = y
    then x = \log_2(y)

    Note: On a calculator use log(y)/log(2) to get \log_2(y)

    The questions you are asking is 10\times 2^n = 80.
    Simplify this into 2^n = 8.
    Then n = \log_2(8) = 3.
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  3. #3
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    Thanks, sounds complicated!!

    How would I type this into a spreadsheet to give the answer in the box?

    Sorry to be a pain!
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  4. #4
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    If you use something like Excel, then they have the logarithm function.
    I think it is in Insert -> Function -> Math and Trig functions -> log

    Then if you want to see how many times A needs to be doubled for B type

    = log(B/A)/log(2)

    where "/" is division
    and "log(...)" is any log function in Excel.

    It may give you decimals if it is not exactly double.
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  5. #5
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    Wow!

    Awesome stuff, big thanks for your help!!!
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