How can we find fast if a number is divisible by 9?

We just add its digits and test if the result is divisible by 9.

Examples:

Is 126 divisible by 9?
1 + 2 + 6 = 9
9 is divisible by 9, so 126 is also (126 = 9 * 14).

How to calculate the square of a number that ends in 5 fast?

15*15=225
25*25=625
35*35=1225
45*45=2025
55*55=3025

We just delete the last digit of the number (the 5) and multiply this shorter number with the one that is bigger with 1 than it. After that we glue a 25 at the end of the product.

Is there a way to multiply a two digits number with 11 fast?

Multiplying a two digits number with 11 is actually very simple: you just add the two digits and put them in the middle.

23*11=?
We just add 2 and 3 and put the result in the middle:
2+3=5
Now we put 5 between 2 and 3:
23*11=253

n:

11 * n: