Step 1 :V = 80000(1 - 0.15)^t
Step 2 :\frac{80000}{2} = 80000(1 - 0.15)^t
Step 3 :\ln{(\frac{1}{2})} = t \ln{(1 - 0.15)}
Step 4 :t = \frac{\ln{(\frac{1}{2})}}{\ln{(1 - 0.15)}}
Step 5 :25000 = 80000(1 - 0.15)^t'
Step 6 :\frac{25000}{80000} = (1 - 0.15)^{t'}
Step 7 :\ln{(\frac{25000}{80000})} = t' \ln{(1 - 0.15)}
Step 8 :t' = \frac{\ln{(\frac{25000}{80000})}}{\ln{(1 - 0.15)}}