Answer
See below.
Work Step by Step
a) Our equation is: $0.5A_0=A_0e^{-0.35n}\\0.5=e^{-0.35n}\\\ln{0.5}=-0.35n\\n=\ln{0.5}\frac{1}{-0.35}\approx1.98$
b) Our equation is: $0.1A_0=A_0e^{-0.35n}\\0.1=e^{-0.35n}\\\ln{0.1}=-0.35n\\n=\ln{0.1}\frac{1}{-0.35}\approx6.58$
