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

If all the letters of the word MASTER are permuted in all possible ways and words (with or without meaning) thus formed are arranged in dictionary order, then the rank of the word MASTER is

If Set $A$ contains 8 elements, then number of subsets of $A$ which contain at least 6 elements is

The number of different permutations that can be formed by taking 4 letters at a time from the letters of the word 'REPETITION' is