Unrelated note, you can do simple multiplication by 9 with your fingers: place your palms on a table and imagine your fingers are numbered 1-10 from left to right.

4 * 9? Lift up your fourth finger - you have 3 to the left and 6 to the right - answer is 36.

8 * 9? Lift up your eight finger - 7 to the left and 2 to the right - answer is 72.

I hope it's not totally common knowledge thing.