N=1000
we need to clear t of the model
[tex]1000=40\cdot2^{\frac{t}{30}}[/tex][tex]\frac{1000}{40}=2^{\frac{t}{30}}[/tex][tex]25=2^{\frac{t}{30}}[/tex][tex]\frac{t}{30}=\log _225[/tex][tex]\frac{t}{30}=\frac{\log 25}{\log 2}[/tex][tex]t=\frac{\log 25}{\log 2}(30)=139.31[/tex]