Answer
See below.
Work Step by Step
a) Our equation is: $0.5=1-e^{-0.15t}\\0.5=e^{-0.15t}\\\ln{0.5}=-0.1t\\t=\ln{0.5}\frac{1}{-0.15}\approx4.62$
b) Our equation is: $0.8=1-e^{-0.15t}\\0.2=e^{-0.15t}\\\ln{0.2}=-0.1t\\t=\ln{0.2}\frac{1}{-0.15}\approx10.73$
