$A$ and $B$ can separately complete a piece of work in $20$ days and $30$ days respectively. They worked together for some time,then $B$ left the work. If $A$ completed the rest of the work in $10$ days,then for how many days did $B$ work?

If $16$ men or $20$ women can do a piece of work in $25$ days,in what time will $28$ men and $15$ women do it? (in days)

If $10$ persons can do a job in $20$ days,then $20$ persons with twice the efficiency can do the same job in (in days)

$7$ men can complete a piece of work in $12$ days. How many additional men will be required to complete double the work in $8$ days?

$10$ men working $6\, \text{h}$ a day can complete a work in $18\, \text{days}$. How many $\text{hours per day}$ must $15$ men work to complete the same work in $12\, \text{days}$?

The work done by a woman in $8\, \text{hours}$ is equal to the work done by a man in $6\, \text{hours}$ and by a boy in $12\, \text{hours}$. If working $6\, \text{hours per day}$,$9$ men can complete a work in $6\, \text{days}$,then in how many $\text{days}$ can $12$ men,$12$ women,and $12$ boys together finish the same work working $8\, \text{hours per day}$?