The formula for combinations is. N - r. Even very thick concrete walls will attenuate sound and may.
More formally the number of k-element subsets or k-combinations of an n-element set.
On the third chair 5-2 people can sit on the chair. Well for the first chair 5 people can sit on it. And is the formula to find the unique permutations otherwise known as combinations of n objects taking r at a time. The permutations formula is P nr n.