I would be very surprised if 90% of vehicles use drum brakes on the rear wheels (at least in the US). Every car that myself, my parents, my wife, her parents, have ever owned (except for one) has had disk brakes on the rear wheels. While I know I'm using proof by anecdote, I'm just saying I would be very surprised if it were even the majority.
Unless your parents are very young, or exclusively drove sports cars their entire life, that's almost certainly not true. Front disc / rear drum was a standard design for a long time.
You're more likely to notice disc brakes -- they are more visible from outside the car and require slightly more frequent maintenance -- so you may not be able to think of many cars with rear drums.
