My solution to Les Reid’s advanced problem #148 :
A humidifier has a circular disk of radius r that slowly rotates. It is partially submerged in a tank of water (the disk is perpendicular to the surface of the water). In order to have the greatest humidifying effect, the total area of the wetted part of the disk that is above the water should be maximized. How high above the surface of the water should the center of the disk be to attain this maximum ?