You've learned combinations: counting selections where order doesn't matter. You know the formula C(n,k) = n! / (k!(n-k)!), why it divides by k!, and the symmetry property C(n,k) = C(n,n-k).
You understand Pascal's Triangle, Pascal's identity, and how the triangle connects to the binomial theorem. You can compute combinations efficiently and recognize when to use them versus permutations.
You've solved problems generating combinations, subsets, and combination sums using backtracking. These patterns appear in interviews and real-world applications. Next section: probability basics.