I used to help my granddaughter when she was in junior high and high school. We’d be working on some homework and she’d say.....how do you know all that? lol The thing that really amazed her was that I could do math in my head before she even got the problem written down on paper. I taught her my shortcuts.
Without hesitating I blurted out "85". The young man next to me asked "how did you know that?" lol