There are lots of tricks like that, but unfortunately I can't remember them. I think if you add the adjacent digits of any multiple of 9 it will equal 9(eg 108 = (1+0)+(0+8)). 7 is another one there was a simple trick to also.
BTW : I didn't get the point of the asian lady at the end of 3 ads for several weeks until I noticed that she was holding 3 chopsticks.
