The sum of the digits in the unit's place of all the $4-$digit numbers formed by using the digits $3, 4, 5,$ and $6$,without repetition,is

How many numbers can be formed from the digits $1, 2, 3, 4$ when the repetition is not allowed?

$P_1$ and $P_2$ are two distinct and intersecting planes. Three non-collinear points lie on $P_1$ and another three non-collinear points lie on $P_2$ (none being on the line of intersection of the planes). Then,the maximum number of tetrahedrons formed using these six points is:

$A$ person $X$ has four notes of $1, 2, 5,$ and $10$ denominations. The number of different sums of money she can form from them is

Everybody in a room shakes hands with everybody else. The total number of handshakes is $66$. The total number of persons in the room is:

The number of ways in which $5$ boys and $3$ girls can be seated in a row so that each girl is between two boys is: