Learning about Drops

[Muuss]

Algebra to know the global chances to see an item to drop is kinda easy since all is in percents, just multiply the chance for the group by the chance of each item of the group, and multiplly it again by 100.
IE, a group with 30% probability contains 2 items, one at 20%, one at 80%.
First item will have 0.3*0.2*100=6% chances to drop
Second item will have 0.3*0.8*100=24% chances to drop
each time the mob spawns.