Sawaal

 Permutations and Combinations Questions

Popular Latest Rated Important Formulae

FACTS  AND  FORMULAE  FOR  PERMUTATIONS  AND  COMBINATIONS  QUESTIONS

 

 

1.  Factorial Notation: Let n be a positive integer. Then, factorial n, denoted n! is defined as: n!=n(n - 1)(n - 2) ... 3.2.1.

Examples : We define 0! = 1.

4! = (4 x 3 x 2 x 1) = 24.

5! = (5 x 4 x 3 x 2 x 1) = 120.

 

2.  Permutations: The different arrangements of a given number of things by taking some or all at a time, are called permutations.

Ex1 : All permutations (or arrangements) made with the letters a, b, c by taking two at a time are (ab, ba, ac, ca, bc, cb).

Ex2 : All permutations made with the letters a, b, c taking all at a time are:( abc, acb, bac, bca, cab, cba)

Number of Permutations: Number of all permutations of n things, taken r at a time, is given by:

Prn=nn-1n-2....n-r+1=n!n-r!

 

Ex : (i) P26=6×5=30   (ii) P37=7×6×5=210

Cor. number of all permutations of n things, taken all at a time = n!.

Important Result: If there are n subjects of which p1 are alike of one kind; p2 are alike of another kind; p3 are alike of third kind and so on and pr are alike of rth kind,

such that p1+p2+...+pr=n

Then, number of permutations of these n objects is :

n!(p1!)×(p2! ).... (pr!)

 

3.  Combinations: Each of the different groups or selections which can be formed by taking some or all of a number of objects is called a combination.

Ex.1 : Suppose we want to select two out of three boys A, B, C. Then, possible selections are AB, BC and CA.

Note that AB and BA represent the same selection.

Ex.2 : All the combinations formed by a, b, c taking ab, bc, ca.

Ex.3 : The only combination that can be formed of three letters a, b, c taken all at a time is abc.

Ex.4 : Various groups of 2 out of four persons A, B, C, D are : AB, AC, AD, BC, BD, CD.

Ex.5 : Note that ab ba are two different permutations but they represent the same combination.

Number of Combinations: The number of all combinations of n things, taken r at a time is:

Crn=n!(r !)(n-r)!=nn-1n-2....to r factorsr!

 

Note : (i)Cnn=1 and C0n =1     (ii)Crn=C(n-r)n

 

Examples : (i) C411=11×10×9×84×3×2×1=330      (ii)C1316=C(16-13)16=C316=560

Q:

How many 3-digit numbers can be formed with the digits 1,4,7,8 and 9 if the digits are not repeated ?

A) 60 B) 26
C) 50 D) 64
 
Answer & Explanation Answer: A) 60

Explanation:

Three digit number will have unit’s, ten’s and hundred’s place.

 

Out of 5 given digits any one can take the unit’s place.

 

This can be done in 5 ways. ...              (i)

 

After filling the unit’s place, any of the four remaining digits can take the ten’s place.

 

This can be done in 4 ways. ...              (ii)

 

After filling in ten’s place, hundred’s place can be filled from any of the three remaining digits.

 

This can be done in 3 ways. ... (iii) 

 

So,by counting principle, the number of 3 digit numbers = 5x 4 x 3 = 60

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 3304
Q:

Find the number of ways to arrange 4 people in groups of 3 at a time where order matters?

A) 20 B) 16
C) 24 D) 36
 
Answer & Explanation Answer: C) 24

Explanation:

P(4,3)= P34= 24

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 3873
Q:

Find the number of ways to arrange 6 items in groups of 4 at a time where order matters?

A) 720 B) 640
C) 740 D) 360
 
Answer & Explanation Answer: D) 360

Explanation:

6P4 = 6! / (6-4)! = 360

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 6161
Q:

Find the number of ways to take 20 objects and arrange them in groups of 5 at a time where order does not matter.?

A) 57090 B) 15540
C) 15504 D) 23670
 
Answer & Explanation Answer: C) 15504

Explanation:

C520 = 15504

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 4993
Q:

Find the number of ways to draw a straight, (suit does not matter) beginning with a 4 and ending with a 8?

A) 1024 B) 1296
C) 1094 D) 1200
 
Answer & Explanation Answer: A) 1024

Explanation:

There are 5 slots.

 

                   __ __ __ __ __

 

The first slot must be a four. There are 4 ways to put a four in the first slot.

 

There are 4 ways to put a five in the second slot, and there are 4 ways to put a six in the third slot. etc.

 

(4)(4)(4)(4)(4) = 1024

 

Therefore there are 1024 different ways to produce the desired hand of cards.

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 4158
Q:

A certain marathon had 50 people running for first prize, second, and third prize.How many ways are there to correctly guess the first, second, and third place winners?

A) 2 B) 1
C) 4 D) 3
 
Answer & Explanation Answer: B) 1

Explanation:

There is 1 way to correctly guess who comes in first, second, and third. There is only one set of first, second and third place winners. You must correctly guess these three people, and there is only one way to do so.

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 6034
Q:

How many number of times will the digit ‘7' be written when listing the integers from 1 to 1000?

A) 243 B) 300
C) 301 D) 290
 
Answer & Explanation Answer: B) 300

Explanation:

7 does not occur in 1000. So we have to count the number of times it appears between 1 and 999. Any number between 1 and 999 can be expressed in the form of xyz where 0 < x, y, z < 9.

 

1. The numbers in which 7 occurs only once. e.g 7, 17, 78, 217, 743 etc

 

This means that 7 is one of the digits and the remaining two digits will be any of the other 9 digits (i.e 0 to 9 with the exception of 7)

 

You have 1*9*9 = 81 such numbers. However, 7 could appear as the first or the second or the third digit. Therefore, there will be 3*81 = 243 numbers (1-digit, 2-digits and 3- digits) in which 7 will appear only once.

 

In each of these numbers, 7 is written once. Therefore, 243 times.

 

 

2. The numbers in which 7 will appear twice. e.g 772 or 377 or 747 or 77

 

In these numbers, one of the digits is not 7 and it can be any of the 9 digits ( 0 to 9 with the exception of 7).

 

There will be 9 such numbers. However, this digit which is not 7 can appear in the first or second or the third place. So there are 3 * 9 = 27 such numbers.

 

In each of these 27 numbers, the digit 7 is written twice. Therefore, 7 is written 54 times.

 

 

3. The number in which 7 appears thrice - 777 - 1 number. 7 is written thrice in it.

 

Therefore, the total number of times the digit 7 is written between 1 and 999 is

 

243 + 54 + 3 = 300

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 6841
Q:

A team of 8 students goes on an excursion, in two cars, of which one can seat 5 and the other only 4. In how many ways can they travel?

A) 126 B) 240
C) 120 D) 260
 
Answer & Explanation Answer: A) 126

Explanation:

There are 8 students and the maximum capacity of the cars together is 9.

 

We may divide the 8 students as follows

 

Case I: 5 students in the first car and 3 in the second Or

 

Case II: 4 students in the first car and 4 in the second

 

Hence,     in Case I: 8 students are divided into groups of 5 and 3 in 8C3 ways.

 

Similarly, in Case II: 8 students are divided into two groups of 4 and 4 in 8C4 ways.

 

Therefore, the total number of ways in which 8 students can travel is

 

8C3+8C4 = 56 + 70 = 126.

Report Error

View Answer Report Error Discuss

Filed Under: Permutations and Combinations

0 5604