$A$ can complete a work in $m$ days and $B$ can complete it in $n$ days. How many days will it take to complete the work if both $A$ and $B$ work together?

$A$ takes twice as much time as $B$ and $C$ takes thrice as much time as $B$ to finish a piece of work. Working together they can finish the work in $12$ days. The number of days needed for $A$ to do the work alone is:

How many $days$ will $1648$ persons take to construct a dam,if $721$ persons can build the same in $48$ 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)

$A, B$ and $C$ can complete a piece of work in $15, 30$ and $40$ days respectively. They started the work together,but $A$ left $2$ days before the completion of the work and $B$ left $4$ days before the completion of the work. In how many days was the work completed?

$A$ track of $100\, m$ can be built by $7\, men$ or $10\, women$ in $70\, days.$ How many days will $14\, men$ and $20\, women$ take to build a track of $600\, m$?