Decisions in number rules | Year 3

01 / LEARN

The idea, step by step

A decision rule might say: if the number is less than 10, add 2; otherwise, subtract 3.

Check the condition first and choose just one branch. A number equal to 10 is not less than 10, so it uses the “otherwise” branch.

The new number may need a different branch. Start at 8: add 2 to get 10. In the next round, 10 uses the other branch and becomes 7.

Do not keep adding just because that was the first action. Explain each choice using the number you have now.

Apply once: below 10 → add 2; otherwise → subtract 3
Input	Below 10?	Action	Result
4	Yes	Add 2	6
10	No	Subtract 3	7
12	No	Subtract 3	9

Apply both actions or keep using the first branch.

Instead: Check the condition each round and use only the matching action.

02 / WORKED EXAMPLES

Example 1

Use the rule above once, starting at 6.

Example 2

Use the rule above twice, starting at 9.

11, then 8.

For “below 10 add 2; otherwise subtract 3”, what happens to 10?

Show answer

It becomes 7.

10 is not below 10. Calculate $10 - 3 = 7$ .

03 / WORKSHEET

6 questions · Work at your own pace.

Read the questions online

Use this rule once: below 10, add 2; otherwise, subtract 3. Start at 5.
Use the same rule once. Start at 13.
Use the same rule once. Start at 10.
Use the same rule twice. Start at 8.
A different rule says: above 20, subtract 5; otherwise, add 5. Apply it once to 20.
A child starts at 12 and adds 2 using the first rule. Explain the mistake.

Read the worked solutions

Use this rule once: below 10, add 2; otherwise, subtract 3. Start at 5.
5 is below 10, so $5 + 2 = 7$ .
7.
Use the same rule once. Start at 13.
13 is not below 10, so $13 - 3 = 10$ .
10.
Use the same rule once. Start at 10.
Equal to 10 uses the otherwise branch: $10 - 3 = 7$ .
7.
Use the same rule twice. Start at 8.
$8 + 2 = 10$ . Check again: $10 - 3 = 7$ .
10, then 7.
A different rule says: above 20, subtract 5; otherwise, add 5. Apply it once to 20.
20 is not above 20. Use $20 + 5 = 25$ .
25.
A child starts at 12 and adds 2 using the first rule. Explain the mistake.
The condition must be checked before choosing an action.
12 is not below 10. Subtract 3 to get 9.

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Explaining resulting patterns

Curriculum reference & lesson scope

Australian Curriculum v9 and SA Curriculum Mathematics

Number sequences; step-by-step rules; decisions; explaining resulting patterns.