Let C be the set of residents who drink coffee and C′ the set who don’t, and so on. So C′ contains 10 percent of the residents, T′ 20 percent, W′ 30 percent, and G′ 40 percent. Because every resident abstains from something, these four segments must cover 100 percent of the population. And because the four segments just total 100 percent, there can be no overlap among them. Hence every resident drinks three of the four beverages, and 100 percent drink liquor.