Let it take x minutes to fill the cistern, then work done by (Pipe A in 5 min. + Pipe B in x min.) = 1
\(\Rightarrow\) \(\frac{5}{20}\)+ \(\frac{X}{25}\)= 1 \(\Rightarrow\) \(\frac{25+4X}{100}\) = 1 \(\Rightarrow\) 25 + 4x = 100 \(\Rightarrow\) 4x = 75 \(\Rightarrow\)x = 18.75 min