$A$ and $B$ can do a piece of work in $20$ days and $12$ days,respectively. $A$ started the work alone and then after $4$ days $B$ joined him until the completion of the work. How long did the work last? (in days)

$A$ and $B$ can do a piece of work in $20$ days and $12$ days respectively. $A$ started the work alone and then after $4$ days $B$ joined him until the completion of the work. How long did the work last? (in days)

If $3$ men and $4$ boys can earn $Rs. 2100$ in $7$ days,and $11$ men and $13$ boys can earn $Rs. 8300$ in $8$ days,in what time will $7$ men and $9$ boys earn $Rs. 11000$? (in days)

Two men can do a piece of work in $x$ days. But $y$ women can do it in $3$ days. Then the ratio of the work done by $1$ man and $1$ woman is

$A$ and $B$ can complete a piece of work in $30 \text{ days}$,$B$ and $C$ can complete the same work in $20 \text{ days}$,and $A$ and $C$ can complete it in $15 \text{ days}$. If all of them work together,the time taken to complete the work will be? (in $days$)

$x$ number of men can finish a piece of work in $30$ days. If there were $6$ men more,the work could be finished in $10$ days less. The actual number of men is