\(\frac{1}{ v }+\frac{1}{ u }=\frac{1}{ f }\)
\(\frac{1}{ v _1}+\frac{1}{-15}=\frac{1}{(-20)}\)
\(\frac{1}{ v _1}=-\frac{1}{20}+\frac{1}{15}\)
\(=\frac{-3+4}{60}\)
\(v _1=60\,cm\)
\(\frac{1}{ v _2}+\frac{1}{(-25)}=\frac{1}{(-20)}\)
\(\frac{1}{ v _2}=\frac{-1}{20}+\frac{1}{25}\)
\(=\frac{-5+4}{100}=\frac{-1}{100}\)
\(v _2=-100\,cm\)
\(d =60+100=160\,cm\)
$\left(\mu_{ r }=1\right)$