1. The problem statement, all variables and given/known data At a certain time, two radioactive sources R and S contain the same number of radioactive nuclei. The half-life is 2 hours for R and 1 hour for S. Calculate: (a) the ratio of the rate of decay of R to that of S at this time. (b) the ratio of the rate of decay of R to that of S after 2 hours. (c) the proportion of the radioactive nuclei in S which have decayed in 2 hours. Answers: (a) 1 : 2, (b) 1 : 1, (c) 75 %. 2. The attempt at a solution I get how to approach (c). At t = 0 hours we have 100 % of S, in 1 hour we'll have 50 % of it and in one more hour we'll have half of that -- 25 %. So in two hours 75 % of S decayed. Though I don't quite understand what is required in (a) and (b). In both cases they decay 1 : 2, since S decays faster. Maybe I miss something.