In 1895, Nikolay Bogdanov-Belsky painted the famous: "Mental Arithmetic. In the Public School of S. Rachinsky." The problem on the blackboard is (10²+11²+12²+13²+14²)/365, can you find a way to solve it with mental arithmetic? pic.twitter.com/zoxgjbjkRM

— Fermat's Library (@fermatslibrary) August 2, 2020