Perkalian dengan 99 : Menghitung Cepat

Ada beberapa cara :

1. XY x 99 = XY.(100 - 1) = XY00 - XY
2. XY x 99 = 9.X|9.X + 9.Y|9.Y = 9.X|9.(X + Y)|9.Y
3. (XY x 9).11 = (9.X|9.Y).11 = 9.X|9.(X + Y)|9.Y
4.

1 x 99 = 99 = (01 - 1)(9 - 0) | (10 - 1)
2 x 99 = 198 = (02 - 1)(9 - 0) | (10 - 2)
5 x 99 = 495 = (05 - 1)(9 - 0) | (10 - 5)

10 x 99 = 990 = (10 - 1)(9 - 1) | (10 - 0)
11 x 99 = 1089 = (11 - 1)(9 - 1) | (10 - 1)
13 x 99 = 1287 = (13 - 1)(9 - 1) | (10 - 3)

24 x 99 = 2376 = (24 - 1)(9 - 2) | (10 - 4)

90 x 99 = 8910 = (90 - 1)(9 - 9) | (10 - 0)

111 x 99 = 10989 = (111 - 1)(9 - 11)(10 - 1)

Rumus Umum :

XY x 99 = [XY - 1] [9 - X] | [10 - Y]

XYZ x 99 = [XYZ - 1] [9 - XY] | [10 - Z]