She goes, if we got here at a quarter after 6 and it's 6:45 that's only 20 minutes.
Thought of another thing...
Ask her how long it's going to take pizza to show up if the guy on the phone says he'll be there in 'half an hour'.
According to her logic, should take 50 minutes. Half an hour should be a lot easier to grasp than 1/4 hour.
Maybe that'll kick her thought process up a notch and cause an 'AHA' moment.