Solve the given Problem by arrow method and write why arrow method is useful to solve problem based on unitary method.

$4$ men can do a piece of work in $4$ Days.How many men will do it in $4$days?

$\frac{3}{5}$ quintal of wheat cost  $\mathrm{Rs}\mathit{.}210$ . Find the cost of $1$ quintal of wheat

A batch of bottles is packed in $25$ boxes with $12$ bottles in each box. If the same batch is packed using $20$ bottles in each box, how many boxes would be filled? (Write the numeral part as the final answer).

If a box of sweets is divided among $24$ children, they will get $5$ sweets each. If the number of the children is reduced by $4$, then each would get $k$ sweets. Find the value of $k.$

A factory requires $42$ machines to produce a given number of articles in $63$ days. How many machines would be required to produce the same number of articles in $54$ days?

Two persons could fit new windows in a house in $3$ days. One of the persons fell ill before the work started. If the job will take now $k$ days, then the value of $k$ is