Two twin brothers share the same birthday. Their father gets them a perfectly rectangular birthday cake, and the brothers decide to split the cake into two equal halves so that they each get to eat the same amount of cake.
However, before they can divide it, their father cuts out a perfectly circular (or more precisely, cylindrical) piece from the cake and eats it.
How can the brothers divide the rest of the cake with exactly one straight-line slice? The slice must be a vertical slice straight down through the cake, and is allowed to pass through the removed circle if needed. The brothers have a ruler and a compass to help them choose where to slice the cake.