Important Formulas: Permutations and Combinations | SSC CGL Tier 2 - Study Material, Online Tests, Previous Year PDF Download

Permutation and Combination Formulas

• Number of all permutations of n things, taken r at a time, is given by
  
• Number of all combinations of n things, taken r at a time, is given by
  

Points to Remember

• Factorial of any negative quantity is not valid.
• If a particular thing can be done in m ways and another thing can be done in n ways, then
  • Either one of the two can be done in m + n ways and
  • Both of them can be done in m × n ways
• 0! = 1
• 1! = 1
• If from the total set of n objects and ‘p₁’ are of one kind and ‘p₂’ and ‘p₃’ and so on …. till pr are others respectively then
  
• ⁿP_n = n!
• ⁿc_n = 1
• ⁿc₀ = 1
• ⁿc_r = ⁿc_(n-r)
• ⁿc₀+ⁿc₁+ⁿc₂+ⁿc₃+…ⁿc_n = 2ⁿ  

Permutation and Combination Formulas- Factorial

n ! = n(n-1)(n-2) …… 1 Eg. – 5! = 5(5-1)(5-2)(5-3)(5-4) = 5(4)(3)(2)(1)

Standard Truths

• 0! = 1
• n! only exists of n >= 0 and doesn’t exist for n < 0
  

Permutations Formulas

Number of ways in which Permutations out of n things r things can be SELECTED & ARRANGED (denoted by ⁿP_r).
ⁿP_r = number of permutations (arrangements) of n things taken r at a time.

Eg.

• Arrangement of Letters/Alphabets to form words with meaning or without meaning.
• Arrangements of balls on a table.

Formulas for Combinations

The number of ways in which r things at a time can be SELECTED from from n things is Combinations (represented by ⁿC_r).. ⁿC_r = Number of combinations (selections) of n things taken r at a time.

• ;  where n ≥ r (n is greater than or equal to r).

Eg.

• Selections for people from total numbers who want to go out on a picnic.
• Filling posts with people
• Selection for a sports team out of available players
• Selection of balls from a bag

Important Properties

Property 1

Number of permutations (or arrangements) of n different things taken all at a time = n!

Property 2

For Objects in which P1 are alike and are of one type, P2 are alike or other different type and P3 are alike or another different type and the rest must be all different, Number of permutations =

Property 3

When repetition is allowed number of permutations of n different things taken r at a time = n × n× n ×… (r times) = n^r  

Property 4

Here, we are counting the number of ways in which k balls can be distributed into n boxes under various conditions. The conditions which are generally asked are

• The balls are either distinct or identical.
• No box can contain more than one ball or any box may contain more than one ball.
• No box can be empty or any box can be empty.

Other Properties

• ⁿP_r = r! × ⁿC_r
• ⁿC_r + ⁿC_r-1 = ⁿ⁺¹C_r
• ⁿC_x = ⁿC_y when x = y or x + y = n
• ⁿC_r = ⁿP_n-r
• r . ⁿC_r = n ^n-1C_r-1  
• For ⁿC_r to be greatest
  (a) if n is even, r = n/2
  (b) if n is odd,