IMC 2014 Q6
6. The shape shown on the right was assembled from three identical copies of one of the smaller shapes below, without gaps or overlaps.
Which smaller shape was used?
Intermediate Mathematical Challenge 2014, Question 6. © UK Mathematics Trust. Original paper on ukmt.org.uk
Answer: E
The large shape consists of 21 small squares, so the required shape is made up of 7 small squares. So A and C may be eliminated. The diagram on the right shows that shape E is as required. It is left to the reader to check that neither B nor D was the shape used.
Solution © UK Mathematics Trust, from the IMC 2014 solutions. Longer versions with investigations: Solutions and investigations (PDF).