What is the missing number of the series 7, X, 21, 31, 43?

Question

What is the missing number of the series 7, X, 21, 31, 43?

1. A. 11
2. B. 12
3. C. 13 (Correct answer)
4. D. 14

Correct Answer

Option C — 13

Detailed Solution & Explanation

The correct answer is 13.

Key Points

• The series is 7, 13, 21, 31, 43. The differences between consecutive terms are: 13−7=6, 21−13=8, 31−21=10, 43−31=12. The differences increase by 2 each time, forming an arithmetic progression (6, 8, 10, 12...).
• So the missing number X = 7 + 6 = 13.