Delhi Police Constable 2014 Paper

A and B can do a work in 6 days
(A + B)’s 1 day work $=\frac{1}{6}$
A can do alone this work in 10 days
A’s 1 day work $=\frac{1}{10}$
B’s 1 day work $=\frac{1}{6}-\frac{1}{10}=\frac{5-3}{30}$
$=\frac{2}{30}=\frac{1}{15}$
Hence, B alone will complete this work in 15 days.