Nuclear Radiation Report: Calculations for effect of distance

Not sure why the values are incorrect. Some calculations and an explanation for how you got there would be very helpful. Thank you

Report Table NU.10: Calculations for Effect of Distance Table view List view Calculations for the effect of distance Counts per minute (cpm) Background cpm Source cpm 10 cm 261 14.67 246.33 30 cm 179 14.67 164.33 14.67 46.33 60 cm 61 90 cm 31 14.67 16.33 120 cm 28 14.67 13.33 25 cm N/A N/A 39.41 50 cm N/A N/A 9.85 (-1 pts) Incorrect. (0/1pts) Ratio: 30cm/60cm 4 (-1 pts) Incorrect