A, B complete a work in 25, 20 days respectively. after working together for 5 days A left. B complete the work in

\par (A+B)'s 5 days work =  5\left [\frac{1}{25}+\frac{1}{20}\right] =  5 \left [\frac{4+5}{100}\right] =  5 \left [ \frac{9}{100}\right] =  \frac{9}{20}

\par Remaining work =  1 - \frac{9}{20} = \frac{11}{20}

\par B can complete \frac{11}{20}  \par work =  \frac{11}{20} \times 20 = 11s