## Conceptual Physics (12th Edition)

At 2 m, double the distance, the count rate is $\frac{1}{2^{2}} = \frac{1}{4}$ of 360 counts/minute, or 90 cpm. At 3 m, triple the distance, the count rate is $\frac{1}{3^{2}} = \frac{1}{9}$ of 360 counts/minute, or 40 cpm.