Complete the series: 7, 13, 21, ___, 43, 57.

Correct answer: 31. The differences are 6, 8, 10, 12, 14, so the missing term is 21 + 10 = 31.

Background: If the pattern is not clear, find the differences between terms.

• 32: 32 does not follow the difference pattern.
• 35: 35 does not follow the difference pattern.
• None of these: “None of these” is incorrect because 31 is correct.

Did you know? Check: 31 + 12 = 43 and 43 + 14 = 57.

Memory trick: Differences go up by 2.