Tripods have **three legs** and are used to stand cameras on.

If you had 18 legs, how many tripods could you make?

This problem involves forming groups of 3. Whether the students use repeated subtraction or repeated addition depends on the approach that they use. If they start with 18 legs and form groups of 3 until no legs are left they are modelling repeated subtraction. They are using repeated addition if they start with one tripod (3 legs) and keep adding groups of 3 until they have 18 in total.

sticks or rods

### Problem

Tripods have three legs and are used to stand cameras on. If you had 18 legs, how many tripods could you make?

### Teaching sequence

- Show the students the tripod – count the legs, discuss its function.
- Read the problem to the class.
- Brainstorm for ways to solve the problem.
- If the students suggest that they would like to use equipment ask them for ideas about what they think would be suitable.
- Allow the students to work on the problem – individually or in small groups.
- Circulate asking the students to explain their solutions.
- Share solutions.

#### Extension to the problem

If you can see 27 legs, how many tripods are there?

#### Other contexts for the problem

Tricycles and wheels

#### Solution

When the 18 tripod legs are aranged in groups of 3 they form 6 tripods.