$A$ contractor undertakes to dig a canal $12 \, km$ long in $350 \, days$ and employs $45$ men. After $200 \, days$ he finds that only $4.5 \, km$ of the canal has been completed. Find the number of extra men he must employ to finish the work in time (in $men$).

$4$ mat-weavers can weave $4$ mats in $4$ days. At the same rate,how many mats would be woven by $8$ mat-weavers in $8$ days?

Bindal can finish a work in $10$ days. Jindal is twice as efficient as Bindal. If they work together,in how many days will the work be completed?

If $A$ and $B$ together can complete a piece of work in $15$ days and $B$ alone in $20$ days,in how many days can $A$ alone complete the work?

$3$ women and $18$ children together take $2$ days to complete a piece of work. How many days will $9$ children alone take to complete the piece of work,if $6$ women alone can complete the piece of work in $3$ days?

$A$ $10\, \text{hectare}$ field is reaped by $2\, \text{men},$ $3\, \text{women},$ and $4\, \text{children}$ together in $10\, \text{days}.$ If the working capabilities of a man, a woman, and a child are in the ratio $5: 4: 2,$ then a $16\, \text{hectare}$ field will be reaped by $6\, \text{men},$ $4\, \text{women},$ and $7\, \text{children}$ in how many days?