To prepare for a math PhD program and the math GRE, a solid understanding of probability is beneficial, as it comprises a significant portion of the exam's additional topics section, accounting for about 25% of the total score. An introductory statistics course that includes probability theory may suffice for basic competence, but taking a dedicated probability course is recommended for deeper understanding and preparation. Many test-takers have found that the GRE includes multiple probability questions, often requiring knowledge of basic combinatorics and concepts like jointly distributed random variables. While statistics questions are less common on the GRE, familiarity with probability can enhance overall performance and is particularly useful for those interested in applied mathematics. Engaging with probability can also be enjoyable and intuitive, making it a worthwhile subject to explore further.