This activity is so simple and yet it requires a good detective-type mind to explore it. The answer can be expressed in very many different ways.

You might lead up to this by using simple examples, focusing on encouraging children to explain how they know what the next number is each time.

Tell me about how you got your answer.

How could you get from this number to the next one?

You could create some similiar examples, or encourage pupils to make up their own. It would be interesting to give only two numbers as a start so that there are many possibilities for extending the sequence. Encourage children to find as many different ways of continuing it as they can.

Use of calculators may help.