The number of straight lines that can be formed by joining $8$ points on a circle is:

The number of nine-digit even natural numbers formed using only the digits $0$ and $1$, such that no two consecutive digits are $0$, is:

There are $6$ multiple choice questions in an examination. How many sequences of answers are possible,if the first $2$ questions have $3$ choices each,the next $2$ have $4$ choices each,and the last $2$ have $5$ choices each?

$4$ books,$1$ each in Chemistry,Physics,Biology,and Mathematics,are to be arranged on a shelf. In how many ways can this be done?

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

In how many ways can $5$ sportsmen be selected from a group of $10$?