求救~~~請問此題過程??

Sol
第一次後濃度=0.25*0.9
第二次後濃度=0.25*0.9^2
…
0.25*0.9^n<0.02
0.9^n<0.02/0.25
0.9^n<0.08
log0.9^n<log0.08
nlog(9/10)<log(8/100)
n(2log3－1)<log8－log100
n<(log8－log100)/(2log3－1)
n<(log100－3log2)/(1－2log3)
n<(2－3*0.3040)/(1－2*0.4771)
n<1.398/0.0458
n<30.52
n<=31