In probability and statistics, the number of ways of selecting r items out of a total of n items could be solved using combination or permutation. Combination is the type of arranging things wherein the order matters and no repetition occurs. The opposite is true for permutations.
Thus, for this type of problem, the appropriate solution would be combination. The equation would be n!/r!(n-r)!. The factorial (!) is solved as follows: for example, 15! would be 15×14×13×12×11×10×9×8×7×6×5×4×3×2×1. Applying this process, we could now determine the answer.
15!/4!(15-4)! = 1,365 ways
