1 problem from Putnam, each rated for difficulty. Solve one to open its discussion.
The William Lowell Putnam Mathematics Competition is a North American math contest for college students, organized by the Mathematical Association of America (MAA). Each year on the first Saturday in December, several thousand US and Canadian students spend 6 hours (in two sittings) trying to solve 12 problems. Individual and team winners (and their schools, in the latter case) get some money and a few minutes of fame.