Probability & Counting

Probability: P(event) = favorable outcomes / total outcomes

Always between 0 and 1. P(not A) = 1 - P(A).

Independent events: P(A and B) = P(A) x P(B)

Mutually exclusive: P(A or B) = P(A) + P(B)

General or: P(A or B) = P(A) + P(B) - P(A and B)

Combinations: Order doesn't matter. "Choose 3 from 10."

C(n,r) = n! / (r! x (n-r)!)

C(10,3) = 120

Permutations: Order matters. "Arrange 3 from 10."

P(n,r) = n! / (n-r)!

P(10,3) = 720

GRE tip: Most GRE probability questions involve simple fractions. If your answer is a complex decimal, you probably made an error.

Reference:

Wikipedia: Probability

https://en.wikipedia.org/wiki/Probability