Railways Notes / Mathematics (Quantitative Aptitude) / Chapter 30
Chapter 30 of 31

Permutation and Combination (Basic)

Arrangement vs selection — the fundamental distinction and formulas for counting problems.

📖 ~4 min read 📐 RRB NTPC / Group D — Mathematics

Core Formulas

ConceptFormula
Factorialn! = n × (n−1) × (n−2) × … × 1
Permutation (arrangement)ⁿPᵣ = n! / (n−r)!
Combination (selection)ⁿCᵣ = n! / [r! × (n−r)!]
Use Permutation when ORDER matters (arranging people, forming words). Use Combination when order does NOT matter (selecting a committee/team).
💡 Example: Arrange 3 letters from "MOBILE" (6 distinct letters) → ⁶P₃ = 6×5×4 = 120 ways
💡 Example: Select 2 men from 5 → ⁵C₂ = 5!/(2!×3!) = 10 ways

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