All radioactive decay are first order process.∴ decay constant.     λ=  0.693t1/2 = 0.69328.1 = 0.0247 year-1(i) where t  = 10 years,         λ = 2.303tlogN0Nt = 10 yearsN0 = 1 microgram      = 1×10-6g, N = ?10 = 2.3030.0247log1×10-6Nlog1×10-6N=10×0.02472.303=0.1072        1 × 10-6N =Antilog 0.1072 = 1.1280      N = 1×10-61.1280 = 0.7842 log(ii)        t =60 years         60 = 2.3030.0247log1×10-6Nlog1×10-6N = 60×0.02472.303 = 0.6435      1×10-6N = 60×0.02472.303 = 0.6435      1×10-6N = Antilog 0.6453 = 4.400         N = 1×10-64.400 = 0.227 μgThus, after 10 years and 60 years, 0.7842 log and 0.227 log of 90Sr will be left as undecayed.