Wednesday, January 13, 2010
Multiply by 5
Multiplying a number by 5 and multiply it by like 10 and then divide by 2.
Remember that (obvious but better remembered) that multiplying a number by 10 means add one zero to the bottom of the number.
16 × 5 = (16 × 10) / 2 = 160 / 2 = 80
Another example:
82 × 5 = (82 × 10) / 2 = 820 / 2 = 410
Again
6840 × 5 = 68400 / 2 = 34200
Multiply by 25
Multiplying a number by 25 you multiply the number by 100 (remember that you must add two zeros to the bottom number) and then divide by 4 (derived from 100 = 25 × 4).
Remember, divide by 4 and divide two to say this because 2 times for 2 × 2 = 4
84 × 25 = 8400 / 4 = 4200 / 2 = 2100
38 × 25 = 3800 / 4 = 1900 / 2 = 950
Multiply by 125
The same method, multiply by 125 means to multiply the number by 1000 and divide by 8 (derived from 1000 = 125 × 8).
Note that 8 × 2x2 = 2 and multiply by 1000 means add 3 zeros at the end of the number.
24 × 125 = 24000 / 8 = 12000 / 4 = 6000 / 2 = 3000
58 × 125 = 58000 / 8 = 29000 / 4 = 14500 / 2 = 7250
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