$A$ and $B$ can complete a piece of work in $8$ days,$B$ and $C$ can do it in $12$ days,and $C$ and $A$ can do it in $8$ days. In how many days can $A, B,$ and $C$ together complete the work?

$6$ men or $12$ women can do a piece of work in $20$ days. In how many days can $8$ men and $16$ women do twice as big as this work?

$6$ men or $12$ women can do a piece of work in $20$ days. In how many days can $8$ men and $16$ women do twice as big as this work?

$A, B$ and $C$ can do a piece of work in $30, 20$ and $10$ days respectively. $A$ is assisted by $B$ on one day and by $C$ on the next day,alternately. How long would the work take to finish? (in days)

$A$ can do a piece of work in $30 \text{ days}$ while $B$ can do it in $40 \text{ days}$. $A$ and $B$ working together can do it in (in $\text{days}$)

Four examiners can examine a certain number of answer papers in $10$ days by working for $5$ hours a day. For how many hours in a day would $2$ examiners have to work in order to examine twice the number of answer papers in $20$ days?