It is funny listening to you young bucks figure this one out. It is such old school, it was before the calculator 
Hold all ten digits out in front of you…….4×9=36.
Hold your fourth digit on your left hand down…….count the first 3 as tens……everything else behind that digit as ones…make sence 
It works with any 9 equation. The brains just took the number 9 one step further. Cool hey? 