Answer:
3.6 hours
Step-by-step explanation:
[tex]D(h) = 25 {e}^{ - 0.4h} \\ \\ \implies \: 6 = 25 {e}^{ - 0.4h} \\ \\ \implies \: \frac{6}{25} = {e}^{ - 0.4h} \\ \\ \implies \: 0.24 = {e}^{ - 0.4h} \\ Taking\: natural\: log\:both\: sides \\ In(0.24) = - 0.4h \: In\: e \\ \\ \implies \: -1.42711635564 = - 0.4h\\(\because\:In\:e=1) \\ \\ \implies \: h= \frac{-1.42711635564}{ - 0.4} \\ \\ \implies \: h = 3.5677908891 \\ \\ \implies \: h = 3.6 \: hours[/tex]