A and B can together complete a job in 49 days. A can alone complete it in 56 days. How long would B alone take to finish the job?
One day work of A + B is $1/49$. One day work of A is $1/56$. So one day work of B, say, $1/y$ = $1/49$ - $1/56$. Solving, we get y = 392 days.