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