Left-to-right subtraction algorithm

Left-to-right subtraction algorithm


To help the reader perform the steps of the left-to-right subtraction algorithm for multi-digit numbers, and to suggest some productive ways of thinking about addition in the process.

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The left-to-right algorithm for subtracting multi-digit numbers feels highly unusual to most people when they learn it for the first time (especially to adults). This packet demonstrates the algorithm, and it suggests that there are some important connections between this algorithm and the ways people think about numbers.

Example 1: No regrouping

This video demonstrates the basic algorithm and demonstrates its relationship to (1) the standard algorithm for subtracting multi-digit numbers, and (2) ways of thinking about subtracting multi-digit numbers.

Example 2: Regrouping

This video demonstrates what happens when we need to regroup in the left-to-right algorithm.

Example 3: Regrouping twice

This video demonstrates multiple regroupings.

Example 4: A difficult computation