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I am assuming that one woman can finish the work in same time as a man.

12 men or 18 women can complete a piece of work in 7 days
so total persons = 18+12 =30
30 people can do the work in 7 days.
1 person can do it in = 7×30 = 210 days

8 men and 4 women = 8+4 =12 persons
12 persons can do it in =  \frac{210}{12}  days = 17.5 days

So 4 men and 8 women can complete the same work in 17.5 days.