Question

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?

50% unchanged and 50% stable
25% unchanged and 75% stable
12.5% unchanged and 87.5% stable
75% unchanged and 25% stable

Answer 1

After two half-lives the remaining radioactive fraction is (1/2)^2 = 1/4 = 25%. So 25% unchanged and 75% has decayed (stable). For a 60-g sample that’s 15 g unchanged and 45 g stable.