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You have a stick of seven interlocking cubes (or a tower of seven Lego blocks). You cannot change the order of the cubes.
You break off a bit of it leaving it in two pieces.
Here is one of the ways in which you can do it:
Here is another way you can do it:
In how many different ways can it be done?
Now try with a stick of eight cubes:
What about with a stick of six cubes?
What do you notice?
Now predict how many ways there will be with five cubes.
Try it! Were you right?
How many ways with 20 cubes?
Will your noticing always be true? Can you create an argument that would convince mathematicians?
This problem encourages learners to connect geometrical and numerical ideas, which will help them to generalise and prove. The task makes a very good introduction to algebraic thinking, however, it also has a very accessible starting point so everyone can begin.
This problem featured in an NRICH Primary webinar in March 2022.
You could start by giving everyone in the group some interlocking cubes (or Lego blocks) and asking them to make a stick (or tower) of seven. Ask them to break their stick into two pieces and hold one piece in each hand. Ask how they have done this and record the various ways on the board. (Try to be guided by the group in terms of how you record. Perhaps learners would like you to sketch the two resulting sticks; perhaps they are happy for numbers to be recorded; some may wish to include an addition sign...) Then enquire if there are any other ways that it could have been done. How will they know that they have found all the different ways?
At this point, it might be useful to record each different solution on a different piece of paper, as you can then move them around the board (or table, or floor) to impose some sort of order on them. (Alternatively, you could gather the sticks/towers themselves.) This will help reveal which, if any, have been omitted. This is the basis of working systematically.
How are you going to record what you have done?
How do you know you have found all the ways of breaking it into two pieces?
Can you see a connection between the total number of cubes and the number of ways you can break the stick into two pieces?
How many ways do you think you can break 20 cubes?
How could you express that "for any number"?
How would you convince a mathematician that this is always going to be true?
Learners might find it helpful to use interlocking cubes or Lego blocks and to record on squared paper with coloured pencils/pens.
If all the cubes are the same colour, a split of 4 and 2 will look the same as a split of 2 and 4. Challenge learners to consider how many ways there are of splitting 6 cubes now. Can they predict how may ways there will be with any number of cubes?
Find a great variety of ways of asking questions which make 8.
Vincent and Tara are making triangles with the class construction set. They have a pile of strips of different lengths. How many different triangles can they make?
Noah saw 12 legs walk by into the Ark. How many creatures did he see?