Trees had to be planted in 9 straight rows with each row having 5 trees. Only had 19 seeds for trees. How to plant 9 rows with 5 trees?

The key for this problem is to intersect as often as you can, and to keep in mind that the seeds do not necessarily have to have the same distance between them.

Draw a 5 pointed star.

                                  A

                                 /  X   

B ---------------------------------------------C

      X             /                          X            /

           X       /                             X         / 

               X                                        X                 

            /     X                                 /       X   

        /            X                           /              X

D------------------------------------------------E

                                X    /    

                                  F

Then connect A-F, B-E, and D-C. Each edge(point): point of intersection is where a seed should be planted.  It's much easier to draw this on paper and then circle the intersected points. Obviously, you should have 19 points.