I guess that I was taught basic mental arithmetic differently. Multiply by 8 and divide by 5, or (it's inverse) is two single instruction cycle opcodes (or whatever my brains equivalent is).

Regularly needing to translate between sane units and US (and occasional British) idiosyncrasies keeps these mental muscles worked enough that it's mostly subconscious now.

I didn't enjoy rote repetition of times tables and drills as a kid, but it's frustrating seeing my daughter being taught to understand multiplication, and learning "strategies", but struggling with mental arithmetic (I mean she tests above grade level, so I'm not worried, it's just a _get off my lawn_ reflex)