Do you believe you don’t have to punch all calculations on a calculator to get your answers? Why don’t you move on with your own mini-calculator in your brain.
lets take a look at multiples of nine.
First list all single digits vertically in ascending order .  Then list all single digits again in descending order. You will notice that you basically formed the multiples of nine from 1-10.
0  9
1  8
2  7
3  6
4  5
5  4
6  3
7  2
8  1
9  0
When the second column is 0 the next number should repeat the first column on the previous number  then add nine for the second column. after these, repeat the cycle.
9  9
10  8
11  7
12  6
13  5
14  4
15  3
16  2
17  1
18  0
18  9
19  8
keep on repeating the cycle and you will end up listing all the multiples of nine.