How can you plant 21 trees in 5 rows with 5 in each row?

In a star pattern

I have just drawn it on paper and it works

One tree in middle

Then five rows centered on that one tree

- each row has two trees one side of the centre,

and two tree on the other

So one row is 2 + 1 + 2 trees

So adding 4 trees to make a row

1 + (5 x 4) = 21