I was doing a question which gave the decay rate of the ancient substance as 13.2dpm. The question asked that assuming living plants now has a disintegration rate of 15.3 dpm, and the half-life of carbon-14 is 5730 years, what is the age of the ancient substance. I went on the internet, and got a question similar to this: Charcoal @ stone Henge has C14 in concentration which produced 8.2dpm/gram normal C14 from a tree is 13.5 dpm/g half life of C14 is 5568 years, what is the age of the charcoal. There was a solution on the website which went something like this: Law of radioactive decay: Radioactive decay at rate that is proportional to the amount of radioactive material present. d/dt (decay(t)) = -kdecay(t) t=0 at the time when stone henge is built decay(0) = 13.5 decay(t) = 13.5 x 10^(-kt) 13.5x10^(-k5568)=0.5(13.5) Then I just got so confused.... Please explain how exactly to start a question regarding radioactive decay and what is disintegration per minute per gram?