The thing about time is that any likely division of an hour that you'd like, there's an easy and even division. /2 = 30, /3 = 20, /4 = 15, /5 = 12, /6 =10, /10 = 6 and vice versa.
It's pretty easy to get in the mind state of thinking a half hour is 30 minutes, or more specifically you can think 1.5 hours * 60 minutes / hour. The hours cancel out and you get 90 minutes.