the half life of actinium is 3.92 sec . starting with 0.1 g of Ac , how much would be left at the end of one minute?

No. of half lives (n) =60sec/3.92 sec= 15.30 = approx 15 
Conc after n half lives[Ac]  =[Ac]_o/2ⁿ
                                        =0.1/2¹⁵  =3*10^-6 g