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Marvelous Old Way of Multiplying
Posted:
Jul 4, 1994 12:24 PM


A friend of mine just told me a clever historical technique for multiplying that he read about. It allows one to multiply just by doubling and halving groups of pebbles.
I'll describe the process using numbers.
Write down the first and second numbers (integers) in your multiplication problem.
Successively halve the second number, rounding down each time.
Next to the list of halves, put the doubles of the first number.
For 9*14 you'll get a table that looks like:
9 14 18 7 36 3 72 1
Now cross out all the rows that have an even number in the right column. The sum of the numbers in the left column is the product.
18 + 36 + 72 = 9*2 + 9*4 + 9*8 = 9*14
I believe the computation was originally performed by doubling and halving the contents of separate bowls of pebbles, "crossing out" by discarding certain bowls of stones, and totaling by pouring all the stones into one bowl.
Happy Independence Day!
Heidi



