Cant help with Q25 but I can shed some light on Q20.
As we know the graph is usually drawn with Force on the Y-axis and extension or in this case Length on the X-axis.
What they've done for Q20 is switched the axis around. To find the strain energy you have to calculate the area under the graph. Since the axis is flipped, you'll need to calculate 1/2 X 0.20 X 25 = 2.5J
Hope that helped