\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
Substitute the values.
\(\frac{1}{v}-\frac{1}{20}=\frac{1}{25}\)
\(\frac{1}{v}=\frac{1}{25}-\frac{1}{20}\)
\(=\frac{4-5}{100}\)
\(v =-100\)
The magnification is given as,
\(m =\frac{ v }{ u }\)
\(=\frac{-100}{-20}\)
\(=5\)