Hello,

**1.*1.**
This has nothing to do with extraordinary technique, but you gotta think about it (I'm confident that some of you know about it)..

$\displaystyle (10+a)(10+b)=10(10+a)+b(10+a)=10(10+a)+10b+ab=10(1 0+a+b)+ab$

So multiplying two such numbers is equivalent to :

- sum one of the numbers with the unit digit of the other one

- multiply it by 10

- add the product of the two unit digits.

$\displaystyle 19 \times 17= ?$

- $\displaystyle 17+9=26$

- $\displaystyle 26 \times 10=260$

- $\displaystyle 7 \times 9=63$

- $\displaystyle 19 \times 17=260+63=323$

I hope this would help at least one person in the future

Enjoy calculations !