2 puzzles from Paul Erdos, each explained in a minute and credited to the original work. Free on Savvy.
In 1933, Esther Klein posed a puzzle to her friends in Budapest: take any five points on a plane, with no three of them in a straight line. She claimed you can always pick four of …
— Esther Klein (posed); Paul Erdos and George Szekeres (proved), Happy ending problem
Can every fraction of the form 4/n, where n is a whole number of 2 or more, always be rewritten as the sum of exactly three unit fractions (fractions with 1 on top)? For example, 4…
— Paul Erdős and Ernst G. Straus, Erdős-Straus conjecture