How can seven trees be planted so that there are six rows of trees in a straight line with each row having three trees? For example, if you plant them the following way you get only four rows of three (two horizontal, one vertical, one diagonal).

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Communicated by Jennifer Vahle ([email protected]).

Extra: With 10 trees, make five rows of four trees each.

Hint: remove one tree from the previous case.

I think the next is 19 trees, nine rows with five in each row.