The intensity of gamma radiation emitted by a radioactive source decreases with distance. A student uses a Geiger-Müller tube and a counter to measure the radiation at different distances.

The relationship between the recorded count rate and the distance from the source is given by the formula:

$$count\; rate=\frac{k}{d^2}$$

where $d$ is the distance from the source in metres (m) and $k$ is a constant.

When the Geiger-Müller tube is placed 0.60 m from the source, the recorded count rate is $75000counts\; per\; minute$.