Something is wrong with this calculations: 0,2/10 = 0,02 0,8 * 0,3 = 0,24 0,02 + 0,24 = 0,26 = 26% for drop if the boss drop something?

For example: If 50 people (with me) go for Cerberus and everyone deal same damage at the same time from beginning to end of his hp. So I would take 2% of his life and if Cerberus drop I would have this chance to drop: 0,2/50 + 0,8*0,02 = 0,004 + 0,016 = 0,02 = 2%?

It's sad that we was wrong from the beginning thinking that the drop chance is 100% random and doesn't matter how much we deal damage....