There are 30 people in my math class. 12 of them have cool dads, 15 of them have cool moms, and 9 of them have cool dads and cool moms. How many people have moms and dads who are both uncool?
+23252 30 set of parents
9 sets of both cool dads and cool moms
12 have cool dads
-- subtract out the 9 that also have cool moms and you get 3 cool dads that aren't married to cool
moms
15 have cool moms
-- subtract out the 9 that also have cool dads and you get 6 cool moms that aren't married to cool dads
9 have both cool dads and cool moms
3 have cool dads but uncool moms
6 have cool moms but uncool dads
This leaves 30 - 9 - 3 - 6 = 12 that have both uncool moms and uncool dads
+539 I have a maybe simpler method:
Add the amount of people with cool parents(27) and subtract the number of people with both(9) to get 18 people with cool parents and 12 people have no cool parents
