As I posted here, a large number of problems discourage students even before they read a question. There are two boys who particularly lack attention, and somehow I happened to care for them. I lured them by promising that if they solved the first eleven problems then I would let them go home. This strategy was surprisingly effective. They started to focus on the problems (although they still lack attention compared to other students), and praising them seemed to make them feel respected. I’m not 100% sure that this strategy is good at all, but I’m sure that forcing them to solve 40 problems is bad.