Concept:
In number series problems, observing the pattern of differences between consecutive terms often helps identify the rule governing the sequence.
Step 1: Compute the differences between consecutive terms.
\[
6 - 2 = 4, \quad 12 - 6 = 6, \quad 20 - 12 = 8, \quad 30 - 20 = 10
\]
Step 2: Identify the pattern in the differences.
The differences form the sequence:
\[
4, 6, 8, 10
\]
This sequence increases by \(2\) each time.
Step 3: Determine the next difference.
\[
10 + 2 = 12
\]
Step 4: Find the next term in the series.
\[
30 + 12 = 42
\]
Thus, the missing number in the sequence is:
\[
\boxed{42}
\]