6 men can complete the electric fitting in a building in 7 days.
Question:

6 men can complete the electric fitting in a building in 7 days. How many days will it take if 21 men do the job?

Solution:

It is given that 6 men can complete a job in 7 days.

$\therefore$ Time taken by 1 man to complete the same job $=(6 \times 7)=42$ days

$\therefore$ Time taken by 21 men to complete the same job $=\frac{42}{21}=2$ days