If $^8C_r = ^8C_{r+2}$,then the value of $^rC_2$ is

Ten students are participating in a race. In how many ways can the first $3$ prizes be won?

There are $12$ members in a group. In how many ways can a team of $6$ members be selected such that a particular member is always included?

The total number of $3$-digit numbers,whose sum of digits is $10$,is

In a certain test,there are $n$ questions. In the test,$2^{n-i}$ students gave wrong answers to at least $i$ questions,where $i = 1, 2, \dots, n$. If the total number of wrong answers given is $2047$,then $n$ is equal to

The number of straight lines that can be formed by joining $20$ points,no three of which are in the same straight line,except $4$ of them which are in the same line.