A and B can together complete a job in 21 days. A can alone complete it in 28 days. How long would B alone take to finish the job?
One day work of A + B is $1/21$. One day work of A is $1/28$. So one day work of B, say, $1/y$ = $1/21$ - $1/28$. Solving, we get y = 84 days.