Subtraction on the Cranmer Abacus – Rule of 50 and 00 Sara Larkin Math Consultant Iowa Educational Services for the Blind and Visually Impaired Resource Used: Millaway, S. M. (2002). Abacus basic competency: A counting method. American Printing House for the Blind.

Rule of 50 • Applies whenever you are moving one bead at a time away from the counting bar, you need to apply the rule of 0, and the rod to the immediate left has only a five bead set. • Move the five bead in the next column to the left away from the counting bar with the left index finger and return the four one beads in the same column toward the counting • Return all 9 beads in the current column towards the counting bar simultaneously

Practice Problems • 73 – 28 = 45 • 623 – 148 = 475 • 8644 – 3865 = 4779 • *6347 – 1348 = 4999

Rule of 00 • Applies whenever you are moving one bead at a time away from the counting bar, you need to apply the rule of 0, and the rod to the immediate left has no beads set. • Move to the next bar to the left that has a bead set on it, push one bead away from the counting bar on that rod with the left index finger and return all of the beads between the left hand the right hand toward the counting bar • Return all 9 beads in the current column towards the counting bar simultaneously

Practice Problems • 700 – 2 = 698 • 423 – 225 = 198 • 805 – 209 = 596 • *2658 – 659 = 1999