This problem provides a context within which to explore fractions and proportionality. Seeking concentrations which can be made in different ways and justifying why some concentrations can't be made at all gives practice on working with equivalent fractions, ratios, factors and multiples.

Begin by introducing the idea of a solution with strength 100000 cells/ml. Make sure everyone is clear about what this means.

Then ask the class to imagine mixing 100ml of this solution with 100ml of pure water. Students could record any working out and their answer on individual whiteboards. Now show students the interactivity, and explain how it can be used to perform a single dilution, then use it to check their answer. Take time to discuss how they got to the correct answer.

Demonstrate that the interactivity can measure multiples of 10ml of liquid, up to 100ml - the scientific context of this could be using a dropper that measures 10ml at a time.

Ask students to come up with questions they would like to explore using the interactivity - some suggested questions appear in the problem. Then allow them some time to investigate, using the interactivity to check the predictions that they make.

Once students are competent at working with solutions created using one dilution, use the interactivity to perform a series of two dilutions. Perhaps start by giving them a couple of concentrations to work out, using individual whiteboards as before, and using the interactivity to check. At the end of the problem there are some suggested concentrations they could be asked to make.

Pairs of students could take it in turns to create a concentration using two dilutions, and then challenge their partner to work out the dilutions they used.

Finally, the problem challenges students to investigate impossible dilutions.

*You can read about one teacher's experience of using this task in the classroom.*

If I combine $a$ ml of solution with $b$ ml of water, how does the concentration change?

What happens when several dilutions are performed one after another?

Does the order in which I do the dilutions matter?

The problem Investigating the Dilution Series uses the same context but explores four steps of dilution instead of just one or two. Exact Dilutions extends these same ideas further.

Mixing Lemonade investigates the strengths of different solutions informally and may provide a useful starting point.