Catriona's Three Triangles
The area of the yellow triangle is 15. What’s the total orange area?
This is a spoiler for Catriona's puzzle, https://twitter.com/Cshearer41/status/1321738616482254849, but the sweetest thing here is the reasoning and that is still for the picking.
Here A, B and C have the degrees of freedom, but there are other ways to construct this. In fact, another one follows!