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# 1.21 Algorithms: column subtraction

Created on 09 August 2018 by ncetm_administrator
Updated on 09 August 2018 by ncetm_administrator

## Introduction

Introduce children to the column algorithm for subtraction calculations, applying the algorithm to a variety of partitioning, reduction and difference contexts for two-digit and three-digit numbers; explore exchange (insufficient quantity to subtract from in a column) in detail.

## Teaching points

• Teaching point 1: One number can be subtracted from another using an algorithm called ‘column subtraction’; the digits of the minuend and subtrahend must be aligned correctly; the algorithm is applied working from the least significant digit (on the right) to the most significant digit (on the left).

• Teaching point 2: If there is an insufficient number of any unit to subtract from in a given column, we must exchange from the column to the left.

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