It is observed that only 0.39% of the original radioactive sample remains undecayed after eight hours. Hence:

(a) t1/2=0.693K=0.6930.693=1hr(b)  Mean life λ=1K=1loge⁡2(c) K=2.303tlog⁡N0N=2.3038log⁡1000.39=0.693=loge⁡2(d) Amount of substance left after n half lives, Nt=N012nwhere N0 is the amount of substance initially present and n is the no. of half livesn= Total time t1/2=30min60min=12∴Nt=1081212=1082