21, 34, 55, 89, 144, ?, 377

Answer: 233

Reason:

• Compute successive differences: 34 − 21 = 13, 55 − 34 = 21, 89 − 55 = 34, 144 − 89 = 55.

• These differences are consecutive Fibonacci numbers (13, 21, 34, 55…), so the next difference is 89.

• Therefore the missing term = 144 + 89 = 233.

• Verification: 377 − 233 = 144, which matches the previous term, confirming the pattern.