this is a great detailed list of helpful math tricks, most of them I didn't know before.

One of the things that got me interested in maths early, was after learning long division, we learned short division. Seemed like a great way to divide, with out without using a page. Then there was calculus.........

I got a big laugh out of tip #4 - using your fingers to multiply 9 times any other single number. It;s just as easy to memorize the values for any single digit numbers.

One trick that did come in handy for me that wasn't on the list is something I used when I had to go through a long table of random 4-digit numbers and pick out the ones that were evenly divisible by 3. It took me a fraction of the time it might have, because I knew that if the sum of a number's digits is divisible by 3, the larger number is, too. So, 1421 - 1+4+2+1=8 but 1422 - 1+4+2+2 = 9.

Yes I thought #4 was ridiculous myself.

using your fingers to multiply 9

Don't be too sceptical. Remember, it's a matter of how you learned it in primary school. My wife does it. I had no clue what she was doing one day...she said "Oh, 9 times 7 is (holding up her cute little hands) 63. She had to double check with her hands stating, that's just how I remember.

I never learned the hand trick. All the other kids around me learned it, but I never liked it. I made up my own trick. For 9 times any one-digit number, take the number, subtract one, and that's your ten's place, then that number plus what equals nine? That would be your one's place.
So...
9x5
5-1=4
4+x=9
4+5=9
45

To calculate the absolute internal sum, just drop nines.
Example:

6597238 / drop 9 and 7+2 and 6+3
58 / drop 8+1
4 ( absolute internal sum)

if you dropped all numbers, it was nine.