To multiply by 99, you multiply by 100-1.
Examples:
23 * 99 = 23 * (100 - 1) = 2300 - 23 = 2200 + (100 - 23) = 2277
55 * 99 = 55 * (100 - 1) = 5500 - 55 = 5400 + (100 - 55) = 5445
87 * 99 = 87 * (100 - 1) = 8700 - 87 = 8600 + (100 - 87) = 8613
Now, you know how to multiply by 99 easily. Do NOT use pen, paper, or calculator and only in you mind, do the following exercises in 8 minutes or less:
1) 5 * 99
2) 14 * 99
3) 29 * 99
4) 34 * 99
5) 47 * 99
6) 56 * 99
7) 63 * 99
8) 78 * 99
9) 81 * 99
10) 95 * 99
Do the Working Memory's Hand Exercise again.
