If the dose rate from a sample of Ra-223 was found to be 20 mSv per hour at 10 micrometres, calculate the dose rate at 30 micrometres. Show your calculations. Give your answer to one decimal place

I think it is 20 mSv x 10 micrometres / 30 micrometres, but I am not sure if I have to convert to metres or not (if not I get 6.6).