If a polygon of $$n$$ sides has 560 diagonals, then $$n=$$

A person writes letters to 6 friends and addresses the corresponding envelopes. In how many ways can the letters be placed in the envelopes so that at least two of them are in the wrong envelopes? Notation $$D_n=n!\left(\sum_\limits{i=0}^n \frac{(-1)^i}{i!}\right)$$

A set contains 11 elements. The number of subsets of the set which contain at most 5 elements is

The value of $${ }^6 P_4+4 \cdot{ }^6 P_3$$ is