Number & Letter Series

Reasoning · 5 min read · Sunitha Rao

Finding the pattern and the missing term.

Series questions ask for the rule behind a sequence. Check differences first: if they are constant the series is arithmetic; if the differences themselves form a pattern, look one level deeper.

Then test ratios for geometric series, and squares or cubes for the classic 1, 4, 9, 16 and 1, 8, 27, 64 patterns. Alternating series interleave two rules, so separate the odd and even positions.

Letter series use position values: A is 1 and Z is 26. Pairs such as AZ, BY, CX move forward from the start and backward from the end simultaneously.

Important points

  • Check first differences, then second differences.
  • Constant ratio means a geometric series.
  • Watch for squares, cubes and prime sequences.
  • In letter series, convert letters to position numbers.

Examples

  1. 2, 6, 12, 20, ? - differences are 4, 6, 8, so the next difference is 10 and the answer is 30.
  2. AZ, BY, CX, ? - the next pair is DW.

Practise Number & Letter Series questions →