You might like to print off this sheet
with the empty shapes and the three triangles. Children could cut out the triangles and work in pairs to try to complete the shapes. They will be practising their visualisation skills as well as using a trial and improvement approach in many cases.
The twenty outlines in this problem have been constructed by keeping two of the pieces the same, which you could use as a talking point with the class. There are about sixteen ways of arranging the big triangle with just one of the others, so there are plenty more shapes to create - pupils could take up this idea by making empty shapes for their partner to fill.