Math magic math: Fast mentally conquer

This book emphasizes mental agility, but I wonder if there was a particular moment or experience that motivated this focus on quick mental calculation.

Some tricks seemed straightforward, but others took me a while to get the hang of. Would love to hear if others struggled or had an 'aha' moment.

I've started trying some of these in shopping or quick calculations, but I'm curious if they really hold up in everyday life or just in theory.