Beadbay: the soroban
Taking away on a soroban
Taking away on a soroban mirrors adding. Take the beads if they are there. If there are too few lower beads, take the five away and give back the small friend. If the rod holds too little, borrow one from the rod on the left and add the big friend.
Try these
Each button sets up a sum and shows the next move as ghost beads.
When the beads are there
9 − 2: two lower beads are on the beam, so pull them away. 57 − 53: take 5 from the tens rod, then 3 from the ones rod.
Small friends, the other way round
The other way round: when there are not enough earth beads to take, take the five away and give the small friend back.
7 − 4: the rod shows five and two, so there are not four lower beads to take. Take the five away and give one back.
Big friends and borrowing
When a rod has too little, borrow one from the rod on the left first. Then add the big friend.
15 − 9: the ones rod holds 5, too little to take 9. Borrow one from the tens rod, then add 1, the big friend of 9.
Both at once
14 − 6: borrow one from the tens rod, then add 4, the big friend of 6. But the ones rod already holds 4, so adding 4 needs a small friend: five down, one off. The rod ends on 8.
A borrow that runs on
100 − 3: the ones rod is empty and so is the tens rod. The borrow has to come from the hundreds: take one hundred, leave nine on the tens rod, and add 7 on the ones rod.
Questions
Can a soroban show a negative number?
Not directly. In ordinary practice sums the running total never drops below nothing, and the sums in the app are made that way.
Is subtraction harder than addition?
It uses the same pairs. Most learners find it just as quick once small friends and big friends are familiar.