In a certain town, 90% of the residents drink coffee, 80% drink tea, 70% drink whiskey, and 60% drink gin.
No one drinks all four.
What percent of the residents drink liquor?
If 90% drink coffee then 10% do not drink coffee, similarly, 20% do not drink tea, 30% do not drink whiskey, and 40% do not drink gin.
For nobody to drink all four beverages each the combined total of the do-not-drink must cover all 100% of the town. But the percentages total exactly 100% which means that every person drinks exactly three beverages!
There are two alcoholic beverages but each person excludes only one beverage. This implies every person in the town drinks at least one sort of alcohol - 100% of the residents drink liquor.