Math basics › Probability and statistics
Probability rules
Not
P(not A) = 1 − P(A). Often the fastest route: “at least one” is 1 − P(none).
Or
P(A or B) = P(A) + P(B) − P(A and B). Take off the overlap, or it gets counted twice.
And
If A and B are independent, P(A and B) = P(A) · P(B). If not, P(A and B) = P(A) · P(B | A).
Given
P(A | B) = P(A and B)/P(B). Shrink the world to just the cases where B happened.