There are $6$ candidates for $3$ posts. In how many ways can the posts be filled?

$^nC_r \div ^nC_{r-1} = $

If $n$ is even and the value of $^nC_r$ is maximum,then $r = $

The greatest possible number of points of intersection of $8$ straight lines and $4$ circles is

$A$ car can hold $2$ people in the front seat and $1$ person in the rear seat. If among $6$ persons $2$ can drive,then the number of ways in which the car can be filled is

The sum of the digits in the unit place of all numbers formed with the help of $3, 4, 5, 6$ taken all at a time is