In the sequence 5, 9, ___, 17, 21, what is the missing number?

• A. 11
• B. 13
• C. 15
• D. 9

Answer: (B)

This question tests recognizing an arithmetic sequence, where each term increases by the same amount. From 5 to 9 the jump is +4. If that same +4 pattern continues, the next term is 13, then 17, then 21. So the missing number is 13. In an arithmetic sequence, the difference between consecutive terms stays constant, which is what makes 13 the correct continuation. Other options would break the steady +4 progression (for example, 11 would be a +2 jump, 15 would be a +6 jump, and 9 would repeat), so they don’t fit the pattern.