Use the formulas you know, and possible rearrangements / substitutions. Let A, B, and C be events or sets. Which of the following are always true?
a) P(A ∪ B) ≤ P(A) + P(B)
b) P(A ∩ B) ≤ P(A) + P(B)
c) P(A ∪ B) ≥ P(A) - P(B)
d) P(A ∩ B) ≥ P(A) - P(B)