A GM counter measures a total count rate of $420\text{ counts/s}$ near a sample. Background radiation count rate is $20\text{ counts/s}$. After $18\text{ minutes}$, the total count rate drops to $70\text{ counts/s}$. What is the half-life $t_{1/2}$ of the sample?

• A$6.0\text{ minutes}$
• B$9.0\text{ minutes}$
• C$4.5\text{ minutes}$
• D$3.0\text{ minutes}$

Choose an answer — or press 1234