1-9 by 9 is easy though.

Take the first number, subtract 1 from it, and that will be your 10's place. Your 1's place with be whatever that first number needs added to it to get to nine.

Example... 7x9. 7-1= 6, so 60. 9-6=3. Answer is 63.

Another fun math trick while we're at it. If you want to know if a number is divisible by 3, just add up all the digits, and if that number is divisible by 3, then so is your original number. 3135...3+1+3+5=12, so 3135 is divisible by 3.