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 the radioactive isotope remains unchanged. After a second half-life, half of that remainder is unchanged: 50% × 50% = 25%. The other 75% has decayed into stable material, so the answer is 25 percent unchanged and 75 percent stable.

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