As with addition, when the user does not have the needed beads to set and clear directly, the indirect method (or synthesis) needs to be used. The problem 45-18=27 uses the indirect method.
The user begins by setting the number 45 on the abacus. She then asks, "Can I clear 10 directly?"
After clearing a 1-bead in the tens column the abacus reads 35 as shown in the photo below.
The user then asks, "Can I clear 8 directly?" There are not beads valuing 8 in the ones column to clear. The user must clear a 1-bead in the tens column because this is the next highest number that can be cleared. The abacus now reads 25.
The user was only trying to clear beads totaling 8 but has cleared beads totaling 10. Therefore, 2 more beads must be set so that the abacus reads 27 as shown in the final photo below.