How many different numbers of $6$ digits can be formed with the digits $3, 1, 7, 0, 9, 5$?

Suppose that $20$ pillars of the same height have been erected along the boundary of a circular stadium. If the top of each pillar is connected by beams to the top of all its non-adjacent pillars,then the total number of beams is:

$A$ person wants to climb a $n$-step staircase using one step or two steps. Let $C_n$ denote the number of ways of climbing the $n$-step staircase. Then $C_{18} + C_{19}$ equals

How many even numbers of four digits can be formed with the digits $0, 1, 2, 3, 4, 5$ and $6$,if no digit is used more than once?

In how many ways can a team of $11$ members be formed from a group of $15$ persons?

How many odd numbers of four digits can be formed with the digits $0, 1, 2, 3, 4, 7$ and $8$?