Series Completion Questions for AFCAT

Number Series — Core Patterns

A number series follows a hidden rule connecting each term to the next. Your job is to find the rule and apply it to find the missing or next term. AFCAT typically uses clean, integer-based patterns — avoid over-complicating. Always check the simplest rule first: constant difference, then constant ratio, then squares or cubes.

• Arithmetic series: constant difference between terms. Example: 3, 7, 11, 15 — difference is 4.
• Geometric series: constant ratio. Example: 2, 6, 18, 54 — ratio is 3.
• Square series: terms are perfect squares, sometimes with an offset. Example: 1, 4, 9, 16, 25.
• Cube series: terms are perfect cubes. Example: 1, 8, 27, 64, 125.
• Difference series: the differences between terms themselves form a pattern. Example: 1, 2, 4, 7, 11 — differences are 1, 2, 3, 4.
• Two-step alternating series: odd-positioned and even-positioned terms follow separate rules.

Alphabet Series — Core Patterns

Alphabet series uses positions of letters (A=1, B=2, ... Z=26). AFCAT questions often involve skipping a fixed number of letters, reversing segments, or alternating vowels and consonants. The fastest method is to assign numbers to letters and look for arithmetic patterns in the positions.

• Assign A=1, B=2, C=3 ... Z=26 and convert the series to numbers first.
• Common pattern: skip 1 letter (A, C, E, G), skip 2 letters (A, D, G, J), skip 3 letters (A, E, I, M).
• Reverse alphabets also appear: Z=1, Y=2, X=3 — watch for descending position patterns.
• Mixed series: every alternate letter moves forward by a different step.
• Position mirror trick: A and Z are mirrors (1+26=27), B and Y (2+25=27), and so on.

Mixed and Alphanumeric Series

These series combine letters and numbers in a single sequence, like A1, C3, E5, G7. AFCAT uses these to test whether you can track two patterns simultaneously — one for letters and one for numbers. Treat the letters and numbers as two separate series running in parallel.

• Split the series into its letter part and number part, solve each independently.
• Letter part and number part may have different step sizes.
• Sometimes a pattern applies to groups of three: position 1, 4, 7 follow one rule; 2, 5, 8 follow another.
• Watch for series where one element increases while the other decreases.

Wrong Term in Series

AFCAT sometimes asks you to find the term that does not fit the pattern. The approach is the same — identify the dominant rule, then check which term breaks it. Do not second-guess yourself; if four out of five terms confirm a rule, the fifth is wrong.

• Establish the pattern using the majority of terms before labeling any term wrong.
• Re-check by substituting the correct value — the series should become clean.
• Common trick: one term is off by exactly 1 or 2 — it looks close but breaks the rule.
• In alphabet wrong-term questions, recheck letter positions carefully — B=2 not 3.

• Jumping to complex patterns (squares, cubes) without first checking simple arithmetic difference — AFCAT rewards the simpler rule 80 percent of the time.
• Confusing A=1 with A=0 when calculating alphabet positions — always use A=1, Z=26.
• In alternating series, treating all terms as one sequence instead of splitting into odd-position and even-position sub-series.
• In wrong-term questions, marking the answer after checking only two or three terms — always verify the entire series before choosing.
• Wasting over 90 seconds on a single series question — if the pattern is not visible in 60 seconds, mark and move on.

• ADGR rule for checking patterns: Arithmetic first, Difference-of-differences second, Geometric third, Roots-and-powers last. Go in order.
• Alphabet number anchor: EJOTY — E=5, J=10, O=15, T=20, Y=25. Memorise this to quickly convert letters to numbers without counting from A.
• Mirror pair trick: Every letter pair that adds to 27 are mirrors (A-Z, B-Y, C-X). Useful for descending alphabet series.
• Two-train analogy for alternating series: imagine two trains on separate tracks — odd positions are train 1, even positions are train 2. Solve each train separately.

• AFCAT typically places 3 to 5 Series Completion questions in the Reasoning section. They appear in the first half of the paper and are usually of easy to moderate difficulty — do not skip them.
• Number series in recent AFCAT papers have favoured difference-of-differences patterns and square-based series over pure geometric. Practice these two the most.
• Alphabet series questions often involve a skip-3 or skip-4 letter pattern combined with a reversal. The EJOTY anchor shortcut saves crucial seconds here.
• Wrong-term questions are slightly harder and may appear once per paper. Budget 75 to 90 seconds for these instead of the usual 45 to 60 seconds.
• If a number series has decimals or fractions in the options, the pattern likely involves division or square roots — do not panic, work backwards from the options provided.