\(\frac{1}{ R }=\frac{1}{15}\)
\(\frac{1}{ f _{\text {eq }}}=\frac{1}{ f _{1}}+\frac{1}{ f _{2}}+\frac{1}{ f _{3}}\)
\(=\frac{1}{15}+\left(\frac{5}{4}-1\right)\left[\frac{-2}{ R }\right]+\frac{1}{15}\)
\(=\frac{1}{15}-\frac{1}{30}+\frac{1}{15}\)
\(=\frac{2-1+2}{30}\)
\(=\frac{3}{30}=\frac{1}{10}\)
\(=10\)