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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?

For 87.5% decomposition 1/8 is left.
(1/8) = (1/2)^3
Three half lives is the answer

Fot 99.999% decomposition, 10^-5 is left.
Solve 10^-5 = (1/2)^n for the number of half-lives, n. Take the log of each side of the equatiuon for a quick solution.

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