To determine the probability that a child in Mr. Brown's class has either a pet or a sibling (or both), you need to use the principle of inclusion-exclusion. The formula is given by:
\[ P(A \cup B) = P(A) + P(B) - P(A \cap B) \]
where:
- \( P(A) \) is the probability of having a pet,
- \( P(B) \) is the probability of having a sibling,
- \( P(A \cap B) \) is the probability of having both a pet and a sibling.
You would need the specific probabilities from the Venn diagram to compute the total probability. Please provide the specific values for \( P(A) \), \( P(B) \), and \( P(A \cap B) \) to calculate the final probability.