Theory
Permutation implies an arrangement where the order of things is important and includes word formation, number formation, circular permutation, etc. Combination means selection where order is not important and involves the selection of a team, forming geometrical figures, distribution of things, etc.
Factorial = Factorials are defined for natural numbers, not for negative numbers.
n! = n × (n-1) × (n-2) × ... × 3 × 2 × 1

Formula
Permutation
- nPr = n! / (n-r)!
- If, from a set of n items, p are of one kind, and q, r are of other kinds respectively, then nPr = n! / (p! × q! × r!).
- nPn = n!
Combination
- nCr = n! / [r! × (n-r)!]
- nC0 = 1
- nCn = 1
- nCr = nCn - r
- nCa = nCb ⇒ a = b ⇒ a+b = n
- nC0 + nC1+ nC2+ nC3+ ...............+ nCn = 2n
Solved Examples
Question for Tips & Tricks: Permutation & Combination
Try yourself:In how many ways the letters of the word ‘MINIMUM’ be arranged taking all the letters?
Explanation
MINIMUM contains 7 letters, so total 7! ways. But it contains 2 I’s and 3 M’s so divide by 2! And 3!
So ways 7!/(2! * 3!) = 7*6*5*4*3*2*1 / 2*1*3*2*1 = 420
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Question for Tips & Tricks: Permutation & Combination
Try yourself:In how many ways 5 rings can be worn on 3 fingers?
Explanation
Let these 3 circles are 3 fingers For 1st finger we have 5 choices, for second finger we have 4 choices left of rings, for third finger we have 3 choices left.
So total 5*4*3 = 60 ways
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Question for Tips & Tricks: Permutation & Combination
Try yourself:In how many ways the letters of the word ‘AUTHOR’ be arranged taking all the letters?
Explanation
The word "AUTHOR" consists of 6 distinct letters: A, U, T, H, O, R.
To find the number of ways to arrange these letters, we simply need to find the number of permutations of these 6 distinct letters.
The formula for the number of permutations of nnn distinct objects is n!n!n!, where nnn is the number of objects.
For the word "AUTHOR":
n!=6!=6×5×4×3×2×1=720
Thus, the letters of the word "AUTHOR" can be arranged in 720 different ways.
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