The rule of three gained notoriety for being particularly difficult to explain. Cocker's Arithmetick, the premier textbook in the 17th century, introduces its discussion of the rule of three with the problem "If 4 yards of cloth cost 12 shillings, what will 6 yards cost at that rate?" The rule of three gives the answer to this problem directly; whereas in modern arithmetic, we would solve it by introducing a variable to stand for the cost of 6 yards of cloth, writing down the equation