A 60-g sample of radioactive isotope is measured after two half-lives have passed. What pair of percentages describes the sample after two half-lives?

After one half-life, half of the radioactive isotope remains unchanged. After two half-lives, half of that remainder is left: 50% × 50% = 25%. The other 75% has changed into stable material, so the correct choice is 25% unchanged and 75% stable.

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