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The figure shows an equilateral triangle divided into small equilateral triangles, all equal.
What is the lowest number of small triangles which must now be shaded to produce a figure which has a line of symmetry?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.