A Geiger-M'ller (G-M) tube and counter are used to measure the radiation from a radioactive rock sample.
A student records the cumulative count on the detector every 30 seconds for 150 seconds. The student plots the readings and draws a straight line of best fit.

| Time in s | Counter reading |
|---|---|
| 0 | 0 |
| 30 | 280 |
| 60 | 550 |
| 90 | 810 |
| 120 | 1100 |
| 150 | 1350 |
Calculate the average count rate, in counts/s, using the line of best fit.