This problem encourages children to use number operations and either trial and improvement, or a systematic approach to reach a solution. It would be interesting to share the different ways they use jottings to help during the problem-solving process. The problem could be used to introduce algebraic notation.

What could you try to start with?

What does that tell you about the ages of the other children?

What do you know about David's age compared with Lachlan's age?

Learners could make up another similar problem for a friend to solve.

You could ask questions such as 'what if Lachlan is one year old?' and encourage children to work out the ages of Cassandra and David to see whether all the information holds true. If not, invite learners to suggest what to try next.