Thirty €1 coins are arranged in a circle on a table. Ten of these coins are fake and a little lighter than the 20 real coins. You don’t know where in the circle the fake coins are, but you have been ...

Seven coins have been placed in the “H” shape above. Altogether there are five lines of three, including the diagonals. Your challenge is to place two more coins so that you can make 10 straight lines ...

In 2007 Alexander Shapovalov suggested an unusual coin-weighing problem for the sixth international Kolmogorov math tournament [5]. A judge is presented with 80 coins that all look the same, knowing ...