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Since 9 = 10 − 1, to multiply a number by nine, multiply it by 10 and then subtract the original number from the result. For example, 9 × 27 = 270 − 27 = 243. This method can be adjusted to multiply by eight instead of nine, by doubling the number being subtracted; 8 × 27 = 270 − (2×27) = 270 − 54 = 216.
Subtract the rightmost digit from 10. Subtract the remaining digits from 9. Double the result. Add half of the neighbor to the right, plus 5 if the digit is odd. For the leading zero, subtract 2 from half of the neighbor. Example: 492 × 3 = 1476 Working from right to left: (10 − 2) × 2 + Half of 0 (0) = 16. Write 6, carry 1.
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Byster used to work as a floor trader at the Chicago Mercantile Exchange, but after his cousin, a math teacher in a Chicago area high school, invited him to show the class his shortcuts for doing base 10 arithmetic, Byster quit his job to devote himself to teaching children his methods. [3]
36 represented in chisanbop, where four fingers and a thumb are touching the table and the rest of the digits are raised. The three fingers on the left hand represent 10+10+10 = 30; the thumb and one finger on the right hand represent 5+1=6. Counting from 1 to 20 in Chisanbop. Each finger has a value of one, while the thumb has a value of five.
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