The Ploughman of whom Chaucer remarked, "A worker true and very good was he, Living in perfect peace and charity" protested that riddles were not for simple minds like his, but he would show the good pilgrims, if they willed it, one that he had frequently heard certain clever folk in his own neighbourhood discuss. 

"The lord of the manor in the part of Sussex whence I come hath a plantation of sixteen fair oak trees, and they be so set out that they make twelve rows with four trees in every row. 

Once on a time a man of deep learning, who happened to be travelling in those parts, did say that the sixteen trees might have been so planted that they would make so many as fifteen straight rows, with four trees in every row thereof. 

Can ye show me how this might be? 
Many have doubted that 'twere possible to be done."

The illustration shows one of many ways of forming the twelve rows. 
How can we make fifteen?

See answer