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The Deca Tree

Age 7 to 11
Challenge Level

The Deca Tree

The Deca Tree printable sheet


In the forest there is a Deca Tree.
A Deca Tree has 10 trunks,

tree with 10 trunks

and on each trunk there are 10 branches,

trunk with 10 branches

and on each branch there are 10 twigs,

branch with 10 twigs

and on each twig there are 10 leaves:

twig with 10 leaves

One day a woodcutter came along and cut down one trunk from the tree.
Then he cut off one branch from another trunk of the tree.
Then he cut off one twig from another branch.
Finally he pulled one leaf from another twig.

How many leaves were left on the tree then?
How did you work out the solution?

Click here for a poster of this problem.


Why do this problem?

This problem can give some substance to children's ideas on place value especially when you are introducing or using numbers greater than 1000. 

The problem can also help children to appreciate how big large numbers really are!


Possible approach

You could introduce the problem by telling the class the 'story' about the tree and its trunks, its branches, its twig and its leaves. You may wish to show the group the images in the problem, or you could create your own on the board as you outline what a Deca Tree looks like. 

Introduce the woodcutter and pose the question about the number of leaves left, but try not to say any more at this stage. Give time for children to work in pairs and wander round the room observing. Watch out for different ways of recording. Some pairs might draw rough sketches of the parts of the tree or use some sort of mark to represent them; some might make a table or list of the numbers of trunks, branches, twigs and leaves; others may combine the two and others will record in ways that you would not have predicted. Facilitate a mini plenary to share these different recordings and encourage each pair to say something about how their recording is helping them think about the task. 
You may find that some pairs approach the task by wanting to find out the total number of leaves on a Deca Tree first, then the number of leaves removed by the woodcutter before subtracting the latter from the former. Others may deduct the leaves removed from the tree as they go. 
Depending on your class' experience, you may wish to bring everyone together to work on the calculations together. You could use a second approach to check the solution.


Key questions

Might a picture help?
If there are ten twigs on each branch, how many will there be on ten branches?
How many leaves are there on ten twigs?
How many leaves did the Deca Tree have before the woodcutter came along?
How many leaves did the woodcutter chop off each time?


Possible support

In addition to, or instead of, drawing sketches, it might help some children to make a model of part of the tree. Pipecleaners could work well for this purpose.


Possible extension

Children could be invited to invent a similar tree with different numbers of branches etc., such as a five-branched 'Penta' tree or an eight-branched 'Octa' tree.