For it is now dawn, and my executioner awaits. Shahryar looked thoughtful. Eugene P.Northrop, Riddles in Mathematics , Penguin Books, Harmondsworth 1960. The columns show possible values of the numbers a, b, c, d, and lines show which combinations might occur, based upon estimates of the possible sizes of the numbers involved. Fractions written on lines show how much of the total remains at each stage, and determine the limits on the numbers that follow.
Crosses show cases that lead to impossibilities.