If you look at the help image I attached, what we are given is in RED and what we solve is in BLUE.
First, draw a diagonal in the square. Its length is the diameter, but we don't know the exact value. Since two right triangles have been created, we use Pythagorean Therom to solve for the hypotenuse/diameter:
there are 11 paintings and 9 paintbrushes, added together you have a total of 20 items. There are 9 paintbrushes and 20 items in total which is 9/20. 9/20 as a percentage is 45%