A piece of radioactive substance gives a received count rate of 6000 counts per minute in a detector whose efficiency is known to be 5%. If the sample contains 10^10 atoms, what is the decay constant ( λ ) of this radioactive substance ? No idea how to solve this problem. Any help would be really appreciated. Edit: I am a medical student studying abroad, and the physics teacher doesn't speak English properly. Please believe that I have tried to understand, but after 1 day of looking for information on the internet, I still have no idea how to solve this problem.