Learn how to use Soroban, a japanese version of the Abacus.
What most important about the Soroban or how it is use to calculate number operations, is that it actually does not help you calculate the answer but it just a kind of visual assitance to your numbers. The real calculations and number are actually stored in your mind.
Let me give you a brief example.
8 + 3 = ?
This is how it works. 8 is made up of 5 and 3. or which 8 is 2 less to get 10.
3 is made up of 2 and 1.
with these mental seperation of the numbers, you can see in your mind that u take 2 from "3". put it into "8" which makes 10 and add your balance of 1, which gives you the answer 11.
8 + 3 = 11
same goes for the idea that 6 is made up of 5 and 1.
9 is made up of 5 and 4.
With these break down of numbers you can easily form up your answer.
