1. The problem statement, all variables and given/known data a patient who is undergoing cancer theraphy is given 10 mins of irradiation from a radioactive source that has half life = 2 years. If the same source is to be used for same treatment 2 years later , how long should the patient should be irradiated? the ans is 20 mins .. can someoene teach me how to do this? 2. Relevant equations 3. The attempt at a solution i was told that: at time = 0 (ie. Now) the patient undergoes 10 mins of irradiation. Lets say the amount of radioactivity of the source now is at 100% We are told that it's half life is 2 years which means in 2 years the radioactivity of the source is at 50%. So suppose it is 2 years later if the patient's therapy lasts 10 min - he gets 50% dosage. if the patients therapy goes another 10 min - he gets another 50% dosage So a total of 20 min of therapy results in (50% + 50% = 100% same dosage as he received 2 years earlier) but why if the patient's therapy lasts 10 min - he gets 50% dosage. ??