Six men or nine women can plough a field in 12 days. Four men and six women working together will plough it in how many days?

Since 6 men do as much as 9 women, one man equals 3/2 women. The field is 6 men × 12 days = 72 man-days, and 4 men + 6 women = 4 + 4 = 8 men, so it takes 72/8 = 9 days. Reading 4 men + 6 women as simply a smaller crowd than 6 men + 9 women and scaling 12 days by 15/10 gives 18 days; answering 12 days assumes the new team is as strong as the old one, which it is not.