Copyright © University of Cambridge. All rights reserved.
'Radioactive Triangle' printed from https://nrich.maths.org/
The diagram shows a large equilateral triangle divided by three straight lines into seven regions. The three grey regions are equilateral triangles with sides of length $5\;\mathrm{cm}$ and the central black region is an equilateral triangle with sides of length $2\;\mathrm{cm}$.
What is the side length of the original large triangle?
If you liked this problem,
here is an NRICH task that challenges you to use similar mathematical ideas.