Two different radioactive isotopes decay to 10 percent of their respective original amounts. Isotope A does this in 33 days, while isotope B does this in 43 days. What is the approximate difference in the half-lives of the isotopes?

The time to reach 10% of the original amount is about 3.32 half-lives, since (1/2)^3.32 is approximately 0.10. So the half-life difference is (43 − 33) ÷ 3.32, or about 3 days. The correct choice is B, 3 days.

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