In a town 20% of the people are men, 40% are women and the rest are children. If there are 12000 more children than men, the population of the town is:

Children make up 100 − 20 − 40 = 40% of the town, so they exceed the men by 40 − 20 = 20% of the population. That 20% is 12000, giving a population of 60000. (Then men 12000, women 24000, children 24000 — and 24000 − 12000 = 12000, as required.) Dividing 12000 by the children's own 40% gives 30000 and answers a question about the number of children, not about the GAP; 12000 and 24000 are the men and the women themselves.