1. Subtract directly when you can
To calculate 7 − 2, set 7, then move two earth beads away from the beam. The heaven bead and no earth beads remain active, so the answer is 5.
2. Use friends of 5
If the earth beads you need are not active, exchange with the heaven bead. To subtract 3 from 6, remove 5 and add 2 earth beads. This works because 3 = 5 − 2, leaving 3.
1 + 4 = 5
2 + 3 = 5
3. Borrow with friends of 10
When a rod is too small, subtract 1 from the rod on its left and add the complement on the current rod. For 12 − 7, borrow one ten and add 3 to the ones: 2 + 3 = 5.
1 ↔ 9
2 ↔ 8
3 ↔ 7
4 ↔ 6
5 ↔ 5
Say the move aloud while learning: “minus 7 is minus 10, plus 3.”