You can easily do this in events. The fun of events is making these contraptions physically and having to think about how to design it. Not just entering a % in a box.
What you can do is place a 1x10 in the air, and onRelay, have it fire a projectile downwards. The projectile will spawn randomly somewhere within the area of that 1x10. You can then have two bricks underneath it to represent the chance. A 1x8 and a 1x2 beneath it means an 80% chance of one thing happening, a 20% chance of another happening - and you can activate those with onProjectileHit.
