4.2 Adding and subtracting decimals · Unit 4 Decimals

1 · Teaching slidesuse ← → keys

Teaching this in class? Open the presentation version.

Full-screen dark deck for the projector — bullets reveal one at a time, the videos for this section are built in, plus an interactive tool, quick check questions and speaker notes. One self-contained file: download it and it works with nothing else installed.

Open presentation ↗ All decks

2 · Key words

decimal partThe part of a number after the decimal point.

whole-number partThe part of a number before the decimal point.

mental methodWorking an answer out in your head.

written methodSetting the calculation out on paper, usually in columns.

partitionTo split a number into parts that are easier to work with.

3 · Summary of the sectionrevise from this

In a written method, line up the decimal points — that lines up the place values.
Fill any gaps with zeros: 10 becomes 10.00 before subtracting 4.83.
Mental method 1 — partition. Add the whole-number parts and the decimal parts separately, then combine: 2.3 + 7.8 = (2 + 7) + (0.3 + 0.8) = 9 + 1.1 = 10.1.
Mental method 2 — round and adjust. 6.9 + 12.4 = 7 + 12.4 − 0.1 = 19.3.
For subtraction, round and adjust the other way: 13.3 − 5.8 = 13.3 − 6 + 0.2 = 7.5.
Decimal parts can carry into the units: 0.3 + 0.8 = 1.1, not 0.11.
Always estimate first — 2.3 + 7.8 must be about 10.

PDFPrintable worksheet All 56 questions, space to work PDFWorked answers Full solutions, not just the answer

Video lessons3 videos

Adding Decimals

British Corbettmaths

The column method with the decimal points lined up.

Math Antics - Decimal Arithmetic

American mathantics

Adding and subtracting decimals explained from place value upwards.

Adding and Subtracting Decimals — worked answers

British stretchCorbettmaths

Worked solutions to a full practice set, including subtracting from a whole number.

These players need an internet connection. Everything else on this site works offline.

Adding and subtracting decimals