Why do this problem?
Students are often asked to calculate the average (mean) of sets of whole numbers. But what happens when the numbers vary? This problem
offers students a chance to consolidate their understanding of average as a central measure, representative of the set.
It also offers a chance to rehearse the key mathematical processes of exploring, conjecturing, generalising and justifying.
Introduce the problem by asking the students to imagine they have an infinite supply of 3kg and 8kg weights. Can they find a combination of these weights that has an average of 6kg?
Allow some time for students to work individually or in pairs and then collect solutions. Confirm that there are many correct possibilities but that you would like to focus on the one that involves the least number of weights.
At this point, you may wish to share the image in the problem and invite students to consider how the image can be used to explain why two 3kg weights and three 8kg weights have a mean weight of 6kg.
"If you had other combinations of the 3kg and 8kg weights, what other whole number averages could you make?
What's the smallest? What's the largest?
Can you make all the whole number values in between?"
Students could use a spreadsheet to explore the different combinations, or use the Cuisenaire environment in the problem to create images like the example.
Allow some time for the students to work in pairs, and collect the results on the board for future reference.
Some students may wish to comment on patterns that they notice (e.g. for all possible whole number averages, the number of 3kg and 8kg weights adds up to 5).
"What if you have a different pair of weights? What averages can you now make?"
Encourage students to work in small groups and each choose a different pair of weights (perhaps suggesting that they restrict themselves to weights less than 15kg).
"Share your results with your group. What do you notice? Do your results have anything in common?"
Draw the groups together and share ideas and conjectures. (e.g. students may notice a connection between the number of weights used and the values of those weights)
"Could you make any predictions about what combinations you need to make all possible whole number averages for any pair of weights? Can you use what you notice to find, for example, the combination of 17kg and 57kg weights that have an average of 44kg......of 52kg.......of 21kg.....?"
Encourage students to test and explain their predictions.
What's the smallest average you can make? What's the largest? How do you know?
Can you explain how to make all the whole number averages in between?
Given the original 3kg and 8kg weights, can you find combinations that produce averages of 4.5kg ... of 7.5kg ... of 4.2kg ...of 6.9kg ...? Convince yourself that any (rational) average between 3kg and 8kg is possible.
Students could start by exploring pairs of weights that have a difference of 2kg, then 3kg, then 4kg, and so on, so that patterns emerge as they work systematically.