This is a tricky challenge. You will need to think carefully in order to solve it.
The first picture is exactly as it appears in the problem on the Nrich website.
The second picture is exactly the same but I have added in a dotted line as a hint.
Here is the text that goes with the problem.
I drew this picture by drawing a line from the top right corner of a square to the midpoint of each of the opposite sides. Then I joined these two midpoints with another line.
Can you see four triangles in the square?
What fraction of the area of the square is each of these triangles?