How much of the atoms will remain in the radioactive substance after three half-lives.

The basic formula for nuclear decay relates N is the number of non-decayed nuclei, N is the initial number of nuclei, T is the half-life, t is the measured time:
N = No × 2 ^ (- t / T)
From here, substituting, we get that one eighth of the substance remains, that is, 12.5%

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