The radii of the three circles appear to be in the ratios ...
5 : 10 : 20 = 1 : 2 : 4
Then the areas of the circles will be in the ratios that are the square of these:
1 : 4 : 16
The shaded area is the area of the middle-sized circle, less the area of the smaller circle, so is 4-1 = 3 "ratio units". The ratio of that area to the area of the largest circle is the probability of randomly choosing a point in the shaded area:
