${ }^n{P}_r=3024$ and ${ }^n{C}_r=126$, then $r$ is

Important Questions on Permutation and Combination

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

$n$-digit numbers are formed using only three digits $2, 5$ and $7$. The smallest value of $n$ for which $900$ such distinct numbers can be formed is :

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

The value of $\sum _{r=1}^{10}r·\frac{{}^{n}C_r}{{}^{n}C_{r-1}}$ is equal to

HARD

Mathematics>Algebra>Permutation and Combination>Combinations

In a certain test, there are $n$ questions. In this 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

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

The number of natural numbers less than $7000$ which can be formed by using the digits $0, 1, 3, 7, 9$ (repetition of digits allowed) is equal to:

HARD

Mathematics>Algebra>Permutation and Combination>Combinations

If in a regular polygon the number of diagonals is $54$, then the number of sides of this polygon is:

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

A debate club consists of $6$girls and $4$ boys. A team of $4$ members is to be selected from this club including the selection of a captain (from among these $4$ members) for the team. If the team has to include at most one boy, then the number of ways of selecting the team is

EASY

Mathematics>Algebra>Permutation and Combination>Combinations

On the occasion of Dipawali festival each student of a class sends greeting cards to others. If there are $20$ students in the class, then the total number of greeting cards exchanged by the students is

EASY

Mathematics>Algebra>Permutation and Combination>Combinations

The number of ways of selecting $15$ teams from $15$ men and $15$ women, such that each team consists of a man and a woman is

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

A committee of $11$ member is to be formed from $8$ males and $5$ females. If $m$ is the number of ways the committee is formed with at least $6$ males and $n$ is the number of ways the committee is formed with at least $3$ females, then

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

Suppose that $20$ pillars of the same height have been erected along the boundary of circular stadium. If the top of each pillar has been connected by beams with the top of all its non-adjacent pillars, then the total number of beams is:

EASY

Mathematics>Algebra>Permutation and Combination>Combinations

Consider three boxes, each containing $10$ balls labelled $1, 2, \dots ., 10$. Suppose one ball is randomly drawn from each of the boxes. Denote by $n_i$, the label of the ball drawn from the $i^{\mathrm{t}\mathrm{h}}$ box, $\left(i=1, 2, 3\right)$. Then, the number of ways in which the balls can be chosen such that $n_1<n_2<n_3$ is :

HARD

Mathematics>Algebra>Permutation and Combination>Combinations

If ${\sum _{i=1}^{20}\left(\frac{{ }^{20}{C}_{i-1}}{{ }^{20}{C}_i{+}^{20}{C}_{i-1}}\right)}^3=\frac{k}{21},$ then $k$ equals

HARD

Mathematics>Algebra>Permutation and Combination>Combinations

Let $T_n$ be the number of all possible triangles formed by joining vertices of an $n$-sided regular polygon. If $T_{n+1}-T_n=10$, then the value of $n$ is :

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

The number of integers greater than $6000$ that can be formed, using the digits $3, 5, 6, 7$ and $8$, without repetition is 

HARD

Mathematics>Algebra>Permutation and Combination>Combinations

Let $A=\left\{x_1,x_2,\dots ,x_7\right\} \text{and} B=\left\{y_1,y_2,y_3\right\}$ be two sets containing seven and three distinct elements respectively. Then the total number of functions $f:A\to B$ that are onto, if there exist exactly three elements $x$ in $A$ such that $f\left(x\right)=y_2,$ is equal to:

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

A man $X$ has $7$ friends, $4$ of them are ladies and $3$ are men. His wife $Y$ also has $7$ friends, $3$ of them are ladies and $4$ are men. Assume $X$ and $Y$ have no common friends. Then the total number of ways in which $X$ and $Y$ together can throw a party inviting $3$ ladies and $3$ men, so that $3$ friends of each of $X$ and $Y$ are in this party is:

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

The number of numbers between $2,000$ and $5,000$ that can be formed with the digits $0, 1, 2, 3, 4$ (repetition of digits is not allowed) and are multiple of $3$ is

EASY

Mathematics>Algebra>Permutation and Combination>Combinations

The Number of ways of choosing $10$ objects out of $31$ objects of which $10$ are identical and the remaining $21$ are distinct, is:

EASY

Mathematics>Algebra>Permutation and Combination>Combinations

There are $m$ men and two women participating in a chess tournament. Each participant plays two games with every other participant. If the number of games played by the men between themselves exceeds the number of games played between the men and the women by $84$, then the value of $m$ is :

MEDIUM

Mathematics>Algebra>Permutation and Combination>Combinations

Consider a class of $5$ girls and $7$ boys. The number of different teams consisting of $2$ girls and $3$ boys that can be formed from this class, if there are two specific boys $A$ and $B,$ who refuse to be the members of the same team, is: