Subtraction

Objectives

To subtract using the column method.

To recap how to subtract by regrouping from columns to the left when necessary.

To subtract by counting backwards.

Resources

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

Today's Lesson

Main Activity

Ask your student to recite two more times tables and to answer random questions on these.

Remind your student how to subtract using the column method - Week 7 Expanded Subtraction Methods (YouTube) and Compact Column Subtraction (YouTube) .

Point out that in some columns, the bottom digit may be greater than the top digit. It is not possible to invert the numbers, so the 10s number must be regrouped to provide a 10 for the 1s column. For example:

371 – 153 =

When we set this out in columns, we can see that in the ones column, 3 is greater than 1. We could say aloud, ‘One subtract three, we can’t do. So we need to make the ones number greater by regrouping the next column.’ Regrouping the 10s, 7 is crossed out to become a 6; write the ten in front of the 1 so that it now reads 11. In terms of the regrouping done, 70+1 is now represented as 60+11 to make the subtraction easier (see below). We can then continue with our calculation:


	100s	10s	1s
	3	⁶7	'1
-	1	5	3
	__________________________________________
	2	1	8

Look at the examples on page 76 in Abacus 4 Textbook 1.

These calculations are presented ready for subtraction using the ‘expanded’ method. Your student may prefer to use this method for calculation, rather than the ‘compact’ method presented above. Doing the same calculation using both methods side-by-side may be helpful for your student to make links between the two.

Once your student can recall the concept, ask them to complete the calculations on the page. Though the calculations are written out horizontally, they must be written in column form on the squared paper.

Citations

[1] www.youtube.com [2] www.youtube.com [3] www.youtube.com [4] www.youtube.com