\(C=\frac{2 \pi \varepsilon_0l}{\ln \left(\frac{b}{a}\right)}\)

\(C=\frac{2 \pi \varepsilon_0\left(\frac{20}{100}\right)}{\ln ^2}\)

\(\Delta V=\frac{10 \times 10^{-8} \ln 3}{2 \pi \varepsilon_0\left(\frac{20}{100}\right)}\)

\(\Delta V=\frac{10^{-4} \ln 2}{4 \pi \varepsilon_0}\)

\(\Delta V=\frac{0.1 \ln 2}{4 \pi \varepsilon_0} mV\)