Cool and easy hand multiplication trick for 9’s ?!

How old were you when you found this out?
I was today years old.

So the rule of thumb is,
Going left to right,
Drop a finger according to which number you are multiplying by and the fingers that remain upwards represent a combination of single digits.
So two fingers up on the left side of the down finger and 7 up on the right side is 27.
Seems so simple yet who would’ve thought!