\(\mathrm{v}=\mathrm{f} \lambda \quad \mathrm{f}_2=\frac{2 \mathrm{v}}{2 \mathrm{~L}_2}\)
\(\mathrm{v}=\mathrm{f}_1\left(4 \mathrm{~L}_1\right) \quad \mathrm{f}_2=\frac{\mathrm{v}}{\mathrm{L}_2}\)
\({v}=\mathrm{f}_1\left(4 \mathrm{~L}_1\right)\)
\({f}_2=\frac{\mathrm{v}}{\mathrm{L}_2}\)
\({f}_1=\frac{\mathrm{v}}{4 \mathrm{~L}_1}\)
\({f}_1=\mathrm{f}_2\)
\(\frac{\mathrm{V}}{4 \mathrm{~L}_1}=\frac{\mathrm{V}}{\mathrm{L}_2}\)
\(\Rightarrow \mathrm{L}_2=4 \mathrm{~L}_1\)
\(60=4 \times \mathrm{L}_1\)
\({~L}_1=15 \mathrm{~cm} \)