14 typists typing 6 hours a day can take 12 days to complete the manuscript of a book. How many days will 4 typists, working 7 hours a day, can take to do the same job?

Job is done by $14$ typist working $6$ hours a day in $12$ days.

Job is done by $1$ typist working $6$ hours a day in $(12\times 14)$ days.

Job is done by $1$ typist working $1$ hour a day in $(12\times 14\times 6)$ days.

Job is done by $4$ typist working $1$ hour a day in $\frac{(12\times 14\times 6)}{4}$ days.

Job is done by $4$ typist working $7$ hours a day in $\frac{12\times 14\times 6}{4\times 7}$ days $=36$ days

Hence, the required number of days $=36$