Answer
See below.
Work Step by Step
Our equation is: $2=5e^{-0.4h}\\0.4=e^{-0.4h}\\\ln{0.4}=-0.4h\\h=\ln{0.4}\frac{1}{-0.4}\approx2.29$
, which is approximately $2$ hours and $17$ minutes.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.