An
old chestnut goes like this:
You have ten tree seedlings to plant in your yard. How can you plant them in order to have five rows of four trees each?
Variation: Plant 16 trees in 15 lines of 4.
Answer:
Plant them in the shape of a pentagram, roughly as shown below:
o
o o o o
o o
o
o o
To plant 16 trees in 15 lines of 4, draw a smaller pentagram in the inner pentagon of the first pentagram, and plant 5 trees at the new vertices thus created, and plant the last tree in the center of the whole figure. You'll have 10 lines of 4 that are lines from the pentagrams, and 5 lines of 4 through the center.