This is a pattern-based question where each circle contains two numbers in the outer part, and one in the inner part. The outer numbers are multiplied, and the inner number is the quotient. For example, in the first case:
\[
5 \times 6 = 30, \quad 42 \div 6 = 7 \quad \text{(center number is 7)}.
\]
By applying the same rule to the second circle:
\[
8 \times 2 = 16, \quad 51 \div 2 = 25.5 \quad \text{(center number is 51)}.
\]
Hence, the missing number is 51.
Therefore, the correct answer is "51."