Why do this problem?
is a very challenging way of practising subtraction at the same time as being logical about arranging the numbers. The idea of 'difference' can be hard for children to grasp and this tricky problem is a way of coming to terms with it.
Read the question together and discuss what 'difference' between two numbers means. Try one or two examples like the 5 and 6 given in the problem, making sure that you begin with both the larger and the smaller number. For learners who have met negative numbers it is important to point out that for the purposes of this problem, the difference is always positive. For learners who need quick
success, or find puzzling out problems difficult, you could start with this simpler problem
After the introduction learners could work in pairs on paper or with this interactivity
. Have rough paper available for jottings or use this sheet
for working and recording. If you want a whole class to work on the problem at the same time, this sheet
gives the pyramid of circles, the numbers from 1 to 10 to cut out and leaves space for adding another row of circles should this be required.
The plenary can be used to discuss whether there are different solutions, possibly using the interactivity.
Where could the largest number be? Why?
Does thinking about odd and even numbers help you?
Some children will be motivated to see if different arrangements can be found. They could also try using the numbers 1 -15 on five rows of circles!
Many learners will find it useful to start on this problem
which has the numbers 1 - 6 on three rows of circles. Pupils could use numbered counters that can be moved about if they're not using the interactivity.