Beadbay: the soroban
Multiplying on a soroban
Set the number being multiplied with empty rods to its right. Take its last digit first, multiply it by each digit of the multiplier, add each small product to the right, then clear the digit you used. Work back through the digits and the answer is left on the rods.
Try these
The ghost beads show where each little product goes.
Where things go
Set the number in the middle, with empty rods on its right. Take its last digit first. Add each little times-table answer to the right, then clear that digit.
You need to know your times tables: every step is one fact from them, added on the right rods.
23 × 4, step by step
Set 23 with two empty rods on its right.
- Take the 3 first. 3 × 4 = 12: add 12 on the two rods just right of the 3.
- Clear the 3: it has been used.
- Now the 2. 2 × 4 = 08: add it on the two rods just right of the 2.
- Clear the 2. The rods now show 92.
A two-digit multiplier
Multiply by the tens digit first, then the ones digit, one rod further right. Then clear the digit and move back.
The number of empty rods you leave on the right is one more than the digits in the multiplier.
Why last digit first
Working from the last digit keeps the products out of the way of the digits you have not used yet, so nothing has to be remembered: it is all on the beads.
Questions
Do I need to know my times tables?
Yes, up to 9 × 9. The soroban does the adding; the tables give each small product.
Is this the only way to multiply on a soroban?
No. There are several layouts. This is a common modern one, and the one the app teaches, with the working checked move by move.