Making a Tower Defense Game - Construct 2 Game Development by Example - page 164

Next, let's go to the Projectile on collision with green ball event and add an action. Add
a System action and choose Subtract from , as shown in the following screenshot:
We are going to subtract from ball count. We are going to set the value of the variable to
1 , as shown in the following screenshot. When the projectile destroys the ball, this will
subtract the number of balls we have on the screen.