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.
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.


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.

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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