
Permutation-Combination questions can be challenging if you don’t know the trick to decipher the arrangement of given objects. An important section of Quantitative Aptitude, Permutation-Combination as a concept refers to arrangement of objects in certain order. There can be diverse methods of arranging a set. While permutation is about arranging objects with significance of order in selection, combination is about arranging objects without it. There are usually 2-3 Permutation-Combination questions in exams like CAT. Let's understand Permutation-Combination by definition and examples.
- What is Permutation-Combination?
- Permutation Theorem
- Combination Theorem
- Difference between Permutation and Combination
- CAT Quantitative Aptitude Practice Questions with Solutions
What is Permutation-Combination?
As per the definition, the counting rule or the fundamental counting principle states that if an event has p outcomes and another event has q outcomes, then the number of outcomes when both these events occur together is p x q.
Permutation Formula:
nPr = (n!) / (n-r)!
Combination Formula:
nCr = n!/r!(
Permutation Theorem
A Permutation is an arrangement in a particular order of several things considered a few or all at a time.
Theorem 1: The number of permutations of k different objects taken l at a time, where 0 kPl
Theorem 2: The number of permutations of k different objects taken l at a time, where repetition is allowed, is kr.
Theorem 3: The number of permutations of k objects, where l objects are of the same kind and the rest is all different is k!/l!
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Combination Theorem
A Combination refers to the number of possible arrangements possible for an event. In combination, the order of selection does not matter.
The number of combinations of k items taken at a time is denoted as kCl.
Theorem 4: kCk-l = k!/(k-l)!(k- (k-l))! = k!/(k-l)! l!= kCl
We infer that, selecting l items from k items also means rejecting k-l items.
Theorem 5: kCl = kCm ⇒ l = m or l = k-m, i.e., k = l + m
Theorem 6: kCl = kCl-1 = k+1Cl
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Illustrated Examples of Permuation-Combination
Q 1: Find the value of k such that, kP5 = 42 kP3 , k>4
Solution:
Given that, kP5 = 42 kP3
Applying the formula of permutation,
k (k – 1) (k – 2) (k – 3) (k– 4) = 42 k(k – 1) (k– 2)
According to the question, k > 4 so k(k – 1) (k – 2) ≠ 0
Therefore, by dividing both sides by k(k – 1) (k – 2), we get
(k – 3 (k – 4) = 42 or k2 – 7k – 30 = 0
or k2 – 10k + 3k – 30 or (k – 10) (k + 3) = 0
or k – 10 = 0 or k + 3 = 0 or k = 10 or k = – 3
As k cannot be negative, so k= 10.
Q 2: If kC9 = kC8, find kC17 ?
Solution:
Given, kC9 = kC8
k!/9!(k-9)!= k!/8! (k-8)!
That is, 1/9= 1/k-8
So, k-8 = 9
Hence, k=17 and kC17 = 17C17 = 1
Q 3: In how many ways can 5 balls and 3 boxes be arranged in a row so that no two boxes are together?
Solution:
The 5 balls can be arranged in 5! ways, and to ensure that no two boxes are together, 3 boxes can be put in 6C3 ways.
Total = 5! X 6C3 = 5! X 6!/3!= 4x 5x 2x 3x 4x 5x 6 = 14,400 ways
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Difference between Permutation and Combination
In order to further simplify this concept, we have shared key differences between Permutation and Combination. This table will help you in conceptual clarity.
| Particulars |
Permutation |
Combination |
|---|---|---|
| Definition |
Permutation is an arrangement of all members in a particular order. |
A combination is a selection of members from a collection or group. |
| Represents |
Arrangement |
Selection |
| Order |
Values are arranged in a specific order. |
Values are not arranged in any specific order. |
| Derivation |
There can be multiple permutations from a single combination. |
There can be only single combination from a single permutation. |
| Formula |
nPr = n!/(n-r)! |
nCr = n!/(r!)(n-r)! |
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