Finger Multiplication by Nine
by Paul Horrell
Mathematics Teacher February 2003
The "finger folding" multiplication by 9 can be expanded to include multiplicands up to 99.
Code:The algorithm used is similar to the following: To multiply a two-digit number by 9, ENTER the multiplicand by forming a V between the tens-digit finger and the next finger to its right (SPREAD). ENTER the units digit by FOLDing the units-digit finger.Code:To read the product Hundreds digit: Count the standing fingers before the SPREAD. Tens digit: Moving from left to right, count the fingers from the SPREAD to the FOLD; wrap around if necessary. Units digit: Count the standing fingers to the right of the FOLD.Code:Example: 48x9 Enter (1 2 3 4 _ 5 6 7 F 9 0) Read: Hundreds (1 2 3 4) 4 Tens (5 6 7) 3 Units (9 0) 2 Example with wraparound: 83 x 9 Enter (1 2 F 4 5 6 7 8 _ 9 0) Read: Hundreds (1245 6 78) 7 Tens (9 0 1 2) 4 Units (45 6 7890) 7
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