$A$ can do a work in $15\, days$ and $B$ in $20\, days.$ If they work on it together for $4\, days,$ then the fraction of the work that is left is

$10$ men can complete a piece of work in $15$ days and $15$ women can complete the same work in $12$ days. If all the $10$ men and $15$ women work together,in how many days will the work get completed?

$P$ is twice as good a workman as $Q,$ and together they finish a work in $16$ days. In how many days can it be done by each separately?

$A$ and $B$ together can complete a work in $30$ days. They started together but after $6$ days $A$ left the work and the work is completed by $B$ after $36$ more days. $A$ alone can complete the entire work in how many days?

$A$ takes $10\, \text{days}$ less than the time taken by $B$ to finish a piece of work. If both $A$ and $B$ can do it in $12\, \text{days}$, then the time taken by $B$ alone to finish the work is (in $\text{days}$)

$A$ is $30\%$ more efficient than $B.$ How much time will they,working together,take to complete a job which $A$ alone could have done in $23$ days? (in days)