Answer
$0.58$ hours
Work Step by Step
Plugging in $k=0.25,C=0.9C_0$ into the formula, we get:
$t=-0.25\ln(1-\frac{0.9C_0}{C_0})=-0.25\ln(1-0.9)\approx0.58$ hours
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.