7 men or 21 women can finish a piece of work in 20 days. How many women are required in place of 12 men to complete the same work?

36

7M or 21W can complete a work in 20 days i.e. same amount of work is done by 7M  as  done by 21W in same number of days .

so ratio of  efficiency   M    :    W

                                 3    :    1

that means 1M = 3W

12M = 12 × 3W = 36W

So, 36 women can do same work as done by 12 men