Full question: Suppose that among 1000 households surveyed, 30 have neither an exercise bicycle nor a treadmill, 50 have only an exercise bicycle, and 60 have only a treadmill. How many households have both?
Textbook answer: 860
I am trying to figure out a formula for this kind of problem...
I know that the answer must have come from adding 30 + 50 + 60 and then subtracting this from 1000. Is this because 110 only have one or the other and 30 have neither, so the difference between the universe and those cases must be the case of having both?