7. Decimal operations

Lesson

Let's review how to add and subtract decimals .

Find 4.095-2.609+2.27, writing your answer in decimal form.

Worked Solution

Idea summary

We can use order of operations with decimals, by performing each operation one at a time in a vertical algorithm.

This video looks at story problems involving mass measured to tenths and addition.

Ned trains 3 days a week. On Wednesday he ran 3.4 km, on Friday he ran 7.9 km and on Saturday he ran 2.5 km. How far had he run in total before Saturday?

Worked Solution

Idea summary

To solve addition problems with decimals, we can use a vertical algorithm to add the decimals.

This video uses a story problem that involves units of kilometres measured to hundredths and addition.

During a small earthquake event, two large rocks fell from a cliff. A geologist measured the mass of the two rocks. One rock had a mass of 69.22 kg, and the second rock had a mass of 72.74 kg.

What was the total mass of the two rocks that fell from the cliff?

Worked Solution

Idea summary

To solve addition problems with decimals to hundredths, we can use a vertical algorithm to add the decimals.

This video uses a story problem as the basis for a subtraction question. We also work out which order to subtract in.

Susana pays for a mandarin at the kiosk. The mandarin cost \$1.50 and Susana paid with a \$5 note.

How much change should Susana receive?

Worked Solution

Idea summary

Story problems have key words that help us know if we need to use addition or subtraction. This is not a complete list, so we need to understand the context of the question, to work out whether to add or subtract.

Key words for addition | Key words for subtraction |
---|---|

more | less |

longer | shorter |

later | earlier |

bought | sold |

joined | left |

Add and subtract decimals, with and without digital technologies, and use estimation and rounding to check the reasonableness of answers