Answer to Question #4595 in Discrete Mathematics for Lisa
100 kids are asked about weekend plans. 60 go to a football game, 35 go to the movies and 20 do both. How many kids don't do either?
100 - 60 - 35 = 5, so only 5 kids don't neither go to a football game nor go to the movies. We don't need to take into account that children that do both go to a football game and go to the movies cause they are already included to the previous two groups.