How can you plant seven straight rows of trees with six trees per row such that twenty-one trees are planted in total?

How can you plant nine straight rows of trees with three trees per row such that nine trees are planted in total?

(In reply to re(2): 9 rows, 9 trees by brianjn)

The point was that if the line determined by any pair of trees contains exactly 3 trees, then you have a really nice configuration.  All that do not meet this requirement are somehow ungainly in comparison, although there may be some such which have other symmetries that make them less ungainly -- however, yours is not  one of these.  If you check the official solution, you will find that there is a nice answer that meets the "exactly three" criterion, clearly showing, I think, that the proposer intended that criterion to be met.

Posted by Richard on 2006-10-23 19:59:15