The probable is that which for the most part happens

Today's post is a short math lesson. If you remember probability from middle school you probably want to skip the post. If not, you might find it helpful. I decided to write about probability because so many things in the game are based on RNG(Random Number Generator). Spells/Droprates/Profession Procs all are based on RNG. It's very common to see players say:
"I made 3 Darkmoon Cards and None of them were Nobles, making cards is bad"
"I farmed 30 Stacks of Herbs and got 0 Frost Lotuses, Blizzard nerfed Frost Lotus!"
"I made flasks 20 times and it never procced. Elixir Mastery sucks!"
Or an even better one:
"I prospected 1 stack of Titanium and got 2 epic gems, prospecting titanium is awesome!"
The reply to all of these usually is "Random number generator is random". Quite insightful, eh?

So let's begin our short math lesson. Since WoW uses a random number generator, no previous draw has an effect on the next one. This makes all the calculations in the game very easy.
Let's assume that every time you kill a certain mob, you have a chance to loot 1 to 6 cloth from the mob. How many mobs do you have to kill to get 100 cloth(5 stacks)?
Got an answer? Let's see if you're right. The answer was that you need to kill 28-29 mobs on average.
How did we arrive to this conclusion? First we found the probability of getting cloth from the mob.
Each time we kill a mob we have the chance of getting = 3.5 cloth. 100 divided by 3.5 is 28.57, which was the answer to the problem.
How did I get ? Since we loot 1-6 cloth each time, and the RNG is indeed random and doesn't care about how many cloth we got last time we have to add up all the probabilities of getting cloth:

Does that mean that you are going to get 3.5 cloth each time you kill a mob? No, you might kill 300 mobs and only loot 1 cloth at a time, and somebody else might kill 300 mobs and loot 6 cloth each time. However the more cloth you loot, the more likely you are to reach this number.

You can interpret the graph the following way: 1000 mobs were looted total and the blue line shows how much cloth was looted each time. Notice that it took about 200 tries before the graph reached 3.5 and that the line didn't become completely flat until about 400 mobs were looted.

Hopefully this refreshed your memory about probability. I'll end this thread with some of the % for common drop/proc rates.

Frost Lotus: 5% chance from Northrend Herbs
Nobles Cards: 1/32 chance to get any specific card and 25% chance to get a random Nobles Card.
Prospecting Saronite Ore: 24% chance to get Rare gem (There is something special about prospecting, and there might be a followup post sometime in the future.)
Parrot Cage (Hyacinth Macaw) .07% or roughly 1:1400, which once again doesn't mean that killing 1400 will guarantee one.