1. The problem statement, all variables and given/known data 137Cs has a half life t1/2 = 30.07 years. After 50 years, what fraction of 137Cs will remain? 2. Relevant equations maybe... dN/dλ = λN λ = ln2/t1/2, half life in seconds N = Total mass /(137(1.67x10-17kg)) 3. The attempt at a solution i thought maybe i needed to figure out a decay rate at first but i dont have a total mass. unless i just use the mass of the cesium. so my N will simply equal 1/(1.67x10-17) but that gives me a huge number. 43 million decays per second. so then i thought if i lose 50% in 30 years. then in 20 years, which is 66% the amount of time needed for another half life to pass, then i should lose 66% of the 50% which is only 33%. so in 50 years i lose 33% of the 50% lost in the first 30 years. but then i get lost in my logic and dont know how to proceed. edit: if i take .50 - .33 = .17. then add the .17 to the .50 i get .67. this makes sense to me as after two half lives have passed then .75 of the Cs will be left over.