How to calculate when will I have certain amount of ITEMS based on drop rate

Thanks for the reply, but Im afraid that is not the case. 3 shots at a 16% drop does not equal (3*16%). I found that out here http://wowwiki.wikia.com/wiki/Chance...t_desired_drop
And then, if someone is really good at math here, i would need to extend the question. Imagine you get a chest which drops 3 out of the 6 items. The percentage changes, because once you know the first of the 3 drops, the other two have different percentages, (because first item is 1/6, second is 1/5, third one is 1/4). Following this logic, when will i get 5 pieces of item 1.

Hope someone knows math here better then me