If the half-life of a radioactive substance is 10 days, then the time taken for 75% of the original sample to decay is:
20 days
After one half-life (10 days), 50% of the sample remains and 50% has decayed.
After a second half-life (another 10 days), half of the remaining 50% decays, leaving 25% of the original sample.
So 100% − 25% = 75% has decayed after 2 half-lives = 2 × 10 = 20 days.
Hence, the answer is 20 days.
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