Basic 3 · Mathematics · Number Operations
Adding in Your Head
In this topic you will use quick strategies to add numbers within 100 in your head.
By the end of these notes you should be able to:
- Add by making tens
- Add using doubles and near doubles
- Add tens first, then ones
What you already know
- Adding within 1000
Copy everything that has a blue line on its left into your notebook. The other parts are for reading.
- Strand
- Number
- Sub-strand
- Number Operations
- Indicator
- B3.1.2.3.1 – Use strategies to mentally add and subtract whole numbers within 100
Introduction
Market traders add prices quickly in their heads. You can too, using clever strategies.
1. Strategies for Adding
| Strategy | Example | How |
|---|---|---|
| make ten | 8 + 7 | 8 + 2 = 10, then 10 + 5 = 15 |
| doubles | 25 + 25 | double 25 = 50 |
| near doubles | 25 + 26 | double 25 = 50, plus 1 = 51 |
| tens then ones | 46 + 37 | 46 + 30 = 76, then 76 + 7 = 83 |
| round and adjust | 38 + 19 | 38 + 20 = 58, then take 1 = 57 |
Example
57 + 28 in your head.
- Add the tens: 57 + 20 = 77
- Add the ones: 77 + 8 = 85
Answer: 57 + 28 = 85
New words
- mental
- Done in your head.
- strategy
- A clever way to solve a problem.
- double
- Adding a number to itself.
- round
- To change to the nearest ten.
Summary
- Make ten, use doubles, or add tens then ones.
- Rounding and adjusting makes adding easy.
- Practise to add quickly in your head.
Class activity: Quick Sums
- Your partner says two numbers below 50.
- Add them in your head.
- Say which strategy you used.
You need: Bottle tops, Bundles of sticks, Exercise books
Exercises
Answer in your exercise book first, then check your answers.
1. Double 30 is…
- A. 60
- B. 33
- C. 300
- D. 90
Show answer
Answer: 6030 + 30 = 60.2. 49 + 20 = ?
- A. 69
- B. 59
- C. 79
- D. 89
Show answer
Answer: 69Add 2 tens to 49.3. 15 + 15 is a double.
- A. True
- B. False
Show answer
Answer: TrueIt is 15 added to itself.4. 45 + 46 = 90
- A. True
- B. False
Show answer
Answer: FalseDouble 45 = 90, plus 1 = 91.5. 6 + 4 + 7 = 10 + ______
Show answer
Answer: 76 + 4 makes 10.6. Use tens then ones to add 34 + 45.
Show answer
Answer: 34 + 40 = 74; 74 + 5 = 79Add tens first.
Homework
- Add the prices of two items your family buys in your head. Check on paper.