The probability of unboxing a mythical item is 3%. All of the probability information is shown on the box menu screen. To clarify about the mythical probabilities:
You can calculate the chance of not getting a mythical in a certain number of boxes by taking the chance of not getting a mythical to the power of the number of boxes. Since the chance of getting a mythical is 3% we know the chance of not getting a mythical is 97%. You can then subtract that from 100% to get the chance of getting a mythical in a certain number of boxes.
Here is an example:
Mythical chance percentage = 3%
Non-mythical chance percentage = 100% - 3% = 97%
Non-mythical chance fraction = 97 / 100 = 0.97
Number of boxes = 10
Chance of non-mythical fraction = 0.97 ^ 10 = 0.74
Chance of non-mythical percentage = 0.74 * 100 = 74%
Chance of mythical percentage = 100% - 74% = 26%
I am no mathematician / statistician. My goal with this article is to help convey that for example 100 boxes does not guarantee 3 mythical items.
Examples table of the chance of getting one mythical in a certain number of boxes:
|Number of boxes||Probability of a mythical item (rounded down)|