Probability

CHAPTER 11

11.1 Basic Probability

$P(A) = \dfrac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}}$

11.2 Permutations & Combinations

Type	Formula	When to Use
Permutation $\mathrm{P}^n_r$	$\dfrac{n!}{(n-r)!}$	Order matters
Combination $\mathrm{C}^n_r$	$\dfrac{n!}{r!(n-r)!}$	Order doesn't matter

• "Arrange", "Line up", "Password" → Use P

• "Select", "Choose", "Team" → Use C

11.3 Addition & Multiplication Principles

Principle	Keyword	Formula
Addition	"OR"	$P(A \cup B) = P(A) + P(B)$
Multiplication	"AND"	$P(A \cap B) = P(A) \times P(B)$

$P(\text{at least one}) = 1 - P(\text{none})$

📘 MathsKiller Textbook Series | Chapter 11: Probability