c
\(\frac{1}{\mathrm{f}}=\left(\frac{\mu_{\mathrm{ears}}}{\mu_{\mathrm{alr}}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)

\(\Rightarrow \frac{1}{+20}=\left(\frac{\mu}{1}-1\right)\left(\frac{1}{+15}-\frac{1}{(-30)}\right)\)

\(\Rightarrow \frac{1}{20}=(\mu-1)\left(\frac{3}{30}\right)\)

\(\Rightarrow \mu-1=\frac{1}{2}\)

\(\Rightarrow \mu=1+\frac{1}{2}=\frac{3}{2}=1 \cdot 5\)