The notes below describe a method of engagement based around a technique called complex instruction.

Although this problem is group-worthy, it can, of course, be attempted individually if you wish.

This problem lends itself to collaborative working, both for students who are inexperienced at working in a group and students who are used to working in this way.

The problem involves working out sensible approximations to physical quantities. Making good headway into the task will be difficult and requires good use of teamwork.

Many NRICH tasks have been designed with group work in mind. Here we have gathered together a collection of short articles that outline the merits of collaborative work, together with examples of teachers' classroom practice.

This is an ideal problem for students to tackle in groups of four. Allocating these clear roles (Word, pdf) can help the group to work in a purposeful way - success on this task should be measured by how effectively the members of the group work together as well as by the solutions they reach.

Introduce the four group roles to the class. It may be appropriate, if this is the first time the class have worked in this way, to allocate particular roles to particular students. If the class work in roles over a series of lessons, it is desirable to make sure everyone experiences each role over time.

For suggestions of team-building maths tasks for use with classes unfamiliar with group work, take a look at this article and the accompanying resources.

You may want to make calculators, internet, squared or graph paper, poster paper, and coloured pens available for the Resource Manager in each group to collect. As teacher, you (or the internet) will be a resource containing knowledge of physical data, constants and formulae.

While groups are working divide the board up with the groups names as headings. Listen in on what groups are saying, and use the board to jot down comments and feedback to the students about the way they are working together. This is a good way of highlighting the mathematical behaviours you want to promote, particularly with a challenging task such as this.

You may choose to focus on the way the students are co-operating:

Alternatively, your focus for feedback might be mathematical:

Make sure that while groups are working they are reminded of the need to be ready to present some of their approximations at the end, and that all are aware of how long they have left.

We assume that each group will record their reasoning, assumptions and calculations in preparation for reporting back. There are many ways that groups can report back. Here are just a few suggestions:

- Every group is given a couple of minutes to report back to the whole class, perhaps focussing on explaining two of their approximations. Those listening can seek clarification and ask questions. After each presentation, those listening are invited to offer positive feedback. Finally, those presenting can suggest how the group could have improved their work on the task.
- Everyone makes a poster to put on display at the front of the room, but only a couple of groups are selected to report back to the whole class. Feedback and suggestions can be given in the same way as above. Additionally, students from the groups which don't present can be invited to share at the end anything they did differently.
- Two people from each group move to join an adjacent group. The two "hosts" explain their some of their calculations and reasoning to the two "visitors". The "visitors" act as critical friends, requiring clear mathematical explanations and justifications. The "visitors" then comment on anything they did differently in their own group.

What assumptions have you made?

What other information do you need?

Are there any questions which give an exact answer?

Can you say anything about the accuracy of those answers which aren't exact?

Although this is an approximations question, it is possible to give rigorous bounds on the quantities. Students could be encouraged to give a known upper and lower bound for each quantity, with the focus on providing as tight a bound as possible

Students could also compare their approximations with those of others. Is there a sense in which one of best?

By working in groups with clearly assigned roles we are encouraging students to take responsibility for ensuring that everyone understands before the group moves on.