Subtraction With Decimals

Objectives

To subtract numbers involving decimals using the column method and regrouping if necessary.

Resources

Abacus 4 Textbook 2 | Squared paper | Add and Subtract Decimals (MyMaths) | Subtracting money - Frog Method (YouTube)

Today's Lesson

Main Activity

For a mental warm up, ask some random counting forward and backwards problems in groups of 25s and 50s, but not starting from multiples of 10.

For example:

Count on in 50s starting from 3.

Turn to page 65 in Abacus 4 Textbook 2.

Remind your student of how to subtract with regrouping, when the bottom digit is greater than the top digit. Point out the decimal and show your student how to set out the calculation with all the decimals in a straight, vertical column, giving the decimal points their own square on the paper - watch 'Subtracting Decimals'. Once the decimal points are all in place, tell your student to ignore them and simply do the calculation as if they were not there. Remind your student of the process of regrouping, where a 1 is taken from the digit to the left.

Run through the activities on Add and Subtract Decimals (MyMaths) for more practice if needed.

However, look carefully at Question 8 with your student: 100 - 73.98. To do this calculation using a vertical method would be to invite disaster, as regrouping would need to occur across four place value columns to deal with the first part of the subtraction in the hundredths column:


1000s	100s	10s	1s	.	0.1s	0.01s
	1	0	0	.	0	0
-		7	3	.	9	8

It would be far less error-prone to do this calculation using counting on (frog) on a number line Subtracting money - Frog Method (YouTube). Being able to identify the appropriate method to use in situations like this is a key element of ‘mastery’ of maths.

Citations

[1] www.youtube.com [2] www.youtube.com [3] app.mymaths.co.uk