Soroban unit 5: Mixed friends and long carries
A big friend whose take-away needs a small friend, and carries through several full rods. After this, every move.
When a big friend needs a small friend
Sometimes the take-away part of a big friend cannot be done directly. In 6 + 6 the ones rod cannot take 6, so the big friend says: take away 4 and add 10. But 6 has only one lower bead, so taking away 4 needs a small friend as well. On the soroban this is one fluent movement: add 1, take away the upper bead, then add 10 on the tens rod. The result is 12.
These mixed moves happen when you add 6, 7, 8 or 9 to a rod that shows 5 or more, and in the matching subtractions. There are not many of them, but they are the moves that slow most learners down, so this unit gives them their own practice.
Long carries
A carry can pass through several full rods (995 + 8 = 1,003) and a borrow through several empty ones (1,003 − 8 = 995). Work from the left as always; when a 10 arrives on a full rod, it clears that rod and moves on.
After this unit
You now know every move for adding and subtracting on the soroban, and long columns are these moves one after another. Practice Mixed sums and Columns in Practice, then time yourself in the speed test.