Analogy (numbers) — “13 : 19 :: 21 : ?”. Complete the pair using the same +6 step (next number = previous + 6).

Introduction / Context:
The pair 13 : 19 advances by +6 (13 + 6 = 19). Apply the same constant-difference rule to 21 to find the missing number.

Given Data / Assumptions:

• Rule: x → x + 6.
• We keep arithmetic in integers.

Concept / Approach:
Compute 21 + 6.

Step-by-Step Solution:

Verification / Alternative check:
Constant-difference analogies are common; no contradictory evidence exists in the stem.

Why Other Options Are Wrong:
They do not equal 21 + 6.

Common Pitfalls:
Overcomplicating with primes or products when a simple linear step fits the first pair exactly.

Final Answer:
27