Equating men, women and work:
Suppose 8 men can do a job in 12 days and 20 women can do a job in 10 days. In how many days can 12 men and 15 women do the job.
For solving this again we need concept of man-days:
Work needs = 12 * 8 = 96 man-day or 20*10 = 200 woman-days.
But since work is same quantity we equate both:
96 man-days = 200 woman-days
1 man-day = 2.083 woman-day
Therefore in 1 day, 12 men = 12 * 2.083 = 25 women.
Hence 25+15 women = 40 women work on a job that takes 200 woman-days to complete. So to complete task we need 200 woman-days / 40 woman = 5 days.