How do I use the collision event properly?

So I'm making a game where a tank has to shoot ninja stars and destroy trees but when I use the collision event only one ninja star destroys the tree. What I'm trying to do is that 5 ninja stars destroy the tree instead of just 1, how do I do that?

Create an instance variable on tree to handle the tree "life"

So, change your colision event, instead of destroy the tree on colision, you replace it with:

On Star colision with Tree:

Set tree.life = tree.life -1;

And create an IF:

if tree.life = 0

Destroy Tree

Thank you so much.