Ex:

11 X 99 = 10 89

Let us see how we derived this

Step : 1
We should remember that if we multiply any 2 digit number with another 2 digit number the result will be in 4 digit number

Step : 2
Take the multiplier with 99 which is 11

Step :3
Reduce 1 from 11 which is 10 and keep this as left side of the result

Step : 4
Reduce the result which is 10 from 99 = 89

Step :5
Keep this as right side of the result and club it with left side
10 89

We will see other numbers with 99

Left Side Right Side 11 X 99 = 10 89 (11-1) =10 (99-(11-1)) = 99 – 10 = 89
12 X 99 = 11 88
13 X 99 = 12 87
14 X 99 = 13 86
15 X 99 = 14 85
16 X 99 = 15 84
17 X 99 = 16 83
18 X 99 = 17 82
19 X 99 = 18 81
20 X 99 = 19 80