Peasant multiplication
Peasant multiplication is an old
algorithm for
multiplication. It requires no use of a
multiplication table; however, it requires that the user be able to
divide by 2. The user must also know how to
add.
- Write the two numbers (A and B) you wish to multiply, each at the head of a column.
- Starting with A, divide by 2, discarding any fractions, until there is nothing left to divide. Write the series of results under A.
- Starting with B, keep doubling until you have doubled it as many times as you divided the first number. Write the series of results under B.
- Add up all the numbers in the B-column that are next to an odd number in the A-column. This gives you the result.
Example: 27 times 82
| A-column | B-column | Add this |
| 27 | 82 | 82 |
| 13 | 164 | 164 |
| 6 | 328 | |
| 3 | 656 | 656 |
| 1 | 1312 | 1312 |
| Result: 2214 |
The method works because multiplication is distributive, so:
82 * 27 = 82 * (1*2^0 + 1*2^1 + 0*2^2 + 1*2^3 + 1*2^4) = 82 * (1 + 2 + 8 + 16) = (82 + 164 + 656 + 1312) = 2214.
This method was known to ancient Egyptians as
mediation and duplation, where mediation means
halving one number and duplation means
doubling the other number. It is still used by peasants in some areas, such as Russia.
See also: Multiplication algorithm, Binary numeral system.
External links