Question

A substance is on its fifth half-life. Which approximates the percentages of radioactive and stable isotopes, respectively?

96.9%; 3.1%
3.1%; 96.9%
12.5%; 87.5%
87.5%; 12.5%

Answer 1

3.1%; 96.9%

After 5 half-lives the remaining radioactive fraction is (1/2)^5 = 1/32 ≈ 0.03125 = 3.125%, so about 3.1% radioactive and 96.9% decayed (stable).