Probability of Households with Pets

In a small town, there are 120 households. Out of these households, 30 have a pet cat, 20 have a pet dog, and 10 households have both a cat and a dog.

What is the probability that a randomly selected household has either a pet cat or a pet dog?

Use the formula for calculating the probability:

$$ P(A ext{ or } B) = P(A) + P(B) - P(A ext{ and } B) $$