Yes, it’s from a book called “Uncle John’s Bathroom Reader” and that book was resting on a closed toilet seat in a nation that only kind of uses the metric system in certain situations… however, you’d be surprised how much you can learn after a single visit to any local fast-food joint!
Maybe you’ve never heard of decimal inches. Anything under 1/32" people start using ‘thou’ as in thousandths of an inch.
Subtracting fractions is something you can learn. Children can even do it.