4. Find a common prime factor in each of these pairs of numbers. \( \begin{array}{lll}\text { a) } 21 \text { and } 411 & \text { b) } 77 \text { and } 242 & \text { c) } 56 \text { and } 63\end{array} \)

To find a common prime factor for each pair, let's break it down: a) The prime factors of 21 are 3 and 7, while the prime factors of 411 (which is \(3 \times 137\)) include 3. So, the common prime factor is 3. b) The prime factors of 77 are 7 and 11, and the prime factors of 242 (which is \(2 \times 121\) or \(2 \times 11^2\)) include 11. Thus, the common prime factor is 11. c) The prime factors of 56 are 2 and 7, and for 63, the prime factors are 3 and 7. Hence, the common prime factor is 7. In summary: - a) Common prime factor is 3. - b) Common prime factor is 11. - c) Common prime factor is 7.