Consider a radioactive sample with a half-life of one week. how much of the original sample will be left at the end of the third week?

Radioactive always means first order reaction
the half life of a first order reaction is t(1/2)=ln(2)/k. addtional info is needed to complete the calculations but this will determine the rate constant k. then for a first order reaction: ln[A] = ln[Ao] - kt where [A] is the final amount left after time t (3 weeks) and [Ao] is the inital amount. 

Well to my expert knowledge and my exquisite calculations i think the the negative ion # 412 is the key to sucess

Compound with sulfer  is bright red