The number of ways in which 17 apples can be distributed among four guests such that each guest gets at least 3 apples is .

If a polygon of $n$ sides has 275 diagonals, then $n$ is

The number of positive divisors of 1080 is

If $a_n=\sum\limits_{r=0}^n \frac{1}{{ }^n C_r}$, then $\sum\limits_{r=0}^n \frac{r}{{ }^n C_r}=$