[Charries]

Geiger counter 20cm away measures a count of 6000 s-1. What is count rate 1m away. I get 240 s-1, mark scheme says 1200 s-1. I think this is an easy question, but I am starting to doubt my maths.

[Stonebridge]

If the source is gamma then you would certainly expect the count rate to fall off according to the inverse square law for radiation. Your answer gives the correct value in that case.
The answer provided has used a simple inverse relationship. Forgetting the square!

[DM10]

At 20cm the count is 6000s-1. One meter (100cm) is 5 times bigger than 20cm, this means that the count has to go down by 5 times. 6000/5 = 1200

[Stonebridge]

(Original post by DM10)
At 20cm the count is 6000s-1. One meter (100cm) is 5 times bigger than 20cm, this means that the count has to go down by 5 times. 6000/5 = 1200

Not for the inverse square law for gamma radiation.