Concept:
In number series problems, observe the pattern in the difference between consecutive terms. If the differences follow a recognizable pattern, extend it to determine the next term.
Step 1: Find the differences between consecutive terms.
\[
6 - 2 = 4
\]
\[
12 - 6 = 6
\]
\[
20 - 12 = 8
\]
\[
30 - 20 = 10
\]
Thus, the differences are:
\[
+4, +6, +8, +10
\]
Step 2: Identify the pattern.
The difference increases by \(2\) each time.
Therefore, the next difference is
\[
10 + 2 = 12
\]
Step 3: Find the next term.
\[
30 + 12 = 42
\]
Hence, the missing number is
\[
\boxed{42}
\]