Multiply numbers by 9.
Example: 55 x 9 = 495
Put a zero (0) after 55: 550
Subtract the original number (55) :
550 - 55 = 495

Multiply numbers 1 to 100 by 99.
Example: 55 x 99 =
55 - 1 = 54
99 - 54 = 45
Answer = 5445

Multiply numbers by 999.
Example: 55 x 999 =
55 - 1 = 54
999 - 54 = 945
Answer = 54945

Multiply numbers by 9999.
Example: 55 x 9999 =
55 - 1 = 54
9999 - 54 = 9945
Answer = 549945