=====================================================
Explanation:
The gardener starts at the first tree and then marks every second tree. So they'll skip a tree each time.
The set of trees he marks has the labels {1,3,5,7,9,11,13,15}. Basically it's the set of odd whole numbers between 1 and 15. Or you could notice that we add on 2 each time we need another tree to mark.
Then he goes back to the first tree and marks every third tree. So we have this new set: {1,4,7,10,13}. Start at 1 and add 3 each time to generate a new item.
Union the two sets mentioned and you'll get {1,3,4,5,7,9,10,11,13,15}
We see that the following trees aren't marked: {2,6,8,12,14}
There are 5 items in that last set.
Side note: The trees with labels {1,7,13} have been marked twice. There's a gap of 6 units between each adjacent label.
