Column Subtraction Or Subtraction With Regrouping

Objectives

To understand column subtraction, recognising the need to regroup numbers, if the bottom digit is greater than the top digit.

To understand and apply the concept of place value.

Resources

Abacus 4 Textbook 1 | Squared paper | Counters or beads | Compact Column Subtraction (YouTube) | Expanded Subtraction Methods (YouTube)

Today's Lesson

Main Activity

Turn to page 48 in Abacus 4 Textbook 1.

Look at the examples at the top of the page. These have been written out in the expanded method to demonstrate the concept of regrouping in more than one place value column. The first calculation required regrouping right from the beginning, as the bottom digit in the ones column was larger than the top number. However, the second calculation does not need regrouping in the ones column, as the bottom digit is smaller than the top digit.

Please note, it is very common for a student to try to subtract the smaller digit from the larger digit regardless of whether the bottom number is larger than the top. Point out how this cannot work and demonstrate on paper how you will get two completely different answers by doing the calculation this way and by doing it correctly. Only one answer is correct and that will be the answer to the regrouping calculation. Make ‘top subtract bottom’ your essential handy-hint for subtraction!

Citations

[1] www.youtube.com [2] www.youtube.com