Essential University Physics: Volume 1 (3rd Edition)

We know the following equation: $\frac{1}{2}=e^{\frac{-bt}{2m}}$ Plugging in the value for $b/2m$ and simplifying gives: $\frac{1}{2}=e^{-2.8t} \\ t = \frac{ln(.5)}{-2.8}=\fbox{.248 seconds}$