Column Subtraction

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 | Expanded Subtraction Methods (YouTube)

Today's Lesson

Main Activity

Turn to page 45 in Abacus 4 Textbook 1.

Look at the example 849 - 584 with your student . Explain that this is using the expanded method to show place value.

When subtracting, you must start with the ones column and there is no need to regroup in this question as 9-4=5. However, in the tens column, we are unable to take 80 from 40, so you need to regroup to make the tens number bigger by regrouping the 800 in the 100s column.

The images show this concept modelled using place value counters*: reducing the 800 by one 100 leaves 700 and the 100 is added to the tens column to make the top number bigger than the bottom number: 140, which is greater than 80.

The number is still 849. We started with 800 + 40 + 9 and then changed this to 700 + 140 + 9. This allows us to successfully subtract each of the columns.

Watch Expanded Subtraction Methods (YouTube) to further understand the concept.

Ask your student to try to copy and complete the calculations on the page. They should use the expanded method, to help reinforce the concept.

* You could make your own set of these by writing on counters with a permanent pen.

Citations

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