This puzzle was submitted on Google+ by Mustapha B.Bouzid.
One cuts out an equilateral triangle in two pieces along an altitude. One turns over one of the two pieces, and one slips it in order to obtain the figure. The surface of the equilateral triangle was 600 cm2. What is, in cm2 (and round if necessary), the surface of the covering of the two pieces, in red on the figure?
Try to work out the solution before you read Graeme's solution. Who knows? You may come up with a much more elegant solution than I did.