You don't need paper & pencil or a calculator for
lots of arithmetic. Here's another for you to "do in your head". To multiply any two numbers in the teens:
1. Add the last digit of one number to the other number;
2. Add a zero to the result;
3. Multiply the last digits of your two numbers together;
4. Add result to Step 2.
For example: 17 x 19
1. 17 plus 9 = 26
2. 26 plus zero = 260
3. 7 x 9 = 63
4. 260 plus 63 = 323
Will also work for two numbers in the Twenties,
but Step 2 Result must be multiplied by 2. Thirties by 3, etc.
Easier than "counting cards"!