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how many half-lives have passed if 87.5% of a substance has decomposed? how many if 99.999% has decomposed?

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    Let x be the number of half-lives.

    For 87.5%, 12.5% = 1/8 is left
    2^-x = 1/8
    x = 3

    For 99.999%, 10^-5 is left
    2^-x = 10^-5
    x = 16.6

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