Thread:Monkey D. Luffy;Gomu Gomu No Mi/@comment-25768713-20150614065848/@comment-24808864-20150614173927

The "bracket thing" you are referring to is the combinations formula from combinatorics.

Here is the KA playlist link: https://www.khanacademy.org/math/precalculus/prob_comb/combinations/v/introduction-to-combinations

In the videos Sal refers uses the notation nCk instead of the brackets, but the calculation is effectively the same. It means to imagine you have n objects, and you need tochoose k of them where order does not matter.