"I planted five rows of four Christmas trees each." The man boasted to his boss. The boss looked at him and said, are you saying you planted 20 Christmas trees in one day? No, the man said, I only planted 10 trees. How did he do it?

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The Answer is Just imagine a 5 pointed star, and then plant one tree at each point, and one tree where the sides intersect. There are actually several distinct solutions. All of them can be constructed as follows: Draw a nice long straight line. Draw a second straight line that intersects the first. Draw three more straight lines making sure each line intersects all the lines youve already drawn and avoiding any of the previous points of intersection. That is, no three lines should intersect at the same point. With the first four lines, theres only one topologically distinct configuration, but by varying the position of the fifth line, several different distinct configurations can be created.
Explaination

Explaination:- Just imagine a 5 pointed star, and then plant one tree at each point, and one tree where the sides intersect. There are actually several distinct solutions. All of them can be constructed as follows: Draw a nice long straight line. Draw a second straight line that intersects the first. Draw three more straight lines making sure each line intersects all the lines youve already drawn and avoiding any of the previous points of intersection. That is, no three lines should intersect at the same point. With the first four lines, theres only one topologically distinct configuration, but by varying the position of the fifth line, several different distinct configurations can be created.