Take "miles per hour" for example. I've met plenty of people who can't figure out how long it would take to get from A to B at X mph. They'll deliberate over how they know from running on their treadmill that they run (on average) at 8 mph, and they recall that it usually takes them Y minutes to run Z miles, and then they factor in the diameter of their car's wheels (because surely a car with larger wheels gets there faster for the same mph vs a car with smaller wheels), and finally sprinkle in a bit of multiplication to arrive at their best guestimate.
That is, plenty of people don't realize that "per" means "for each", and that it's not some singular word "milesperhour", but a phrase meaning "miles traveled for every hour spent travelling".
Other fun phrases thrown around without understanding (or with similar words mistakenly swapped in):
