The example pair hands you the cipher. Compare letter positions between the plain word and its code; the gap you find is the key.
Coding types
Shift coding — each letter moves a fixed number forward or back (CAT → DBU is +1). Reverse coding — A becomes Z, B becomes Y (mirror the alphabet). Positional coding — letters are replaced by their numbers (A=1 … Z=26).
- Shift coding — each letter moves a fixed number forward or back (CAT → DBU is +1).
- Reverse coding — A becomes Z, B becomes Y (mirror the alphabet).
- Positional coding — letters are replaced by their numbers (A=1 … Z=26).
How to Approach It
- Lay out the alphabet with numbers — Write A=1 through Z=26 on your rough sheet once at the start of the section. Every coding question after that reduces to simple arithmetic.
- Measure the shift on the example — Line up the plain word against its code letter by letter and find the shift or pattern. Confirm it holds for every letter, not just the first one.
- Name the rule type — Decide whether it is a fixed shift, a mirror (A pairs with Z), or a positional substitution. Identifying the type tells you exactly how to encode the target word.
- Apply it and wrap around — Move each letter by the rule, remembering that going past Z wraps back to A. Encode or decode one letter at a time to avoid slips.
Techniques & Methods
- Position map first — Write A=1 … Z=26 on your rough sheet before the section starts.
- Find and verify the shift — Compare one plain-and-code letter, then confirm the shift on every letter. e.g. CAT → DBU is a +1 shift.
- Mirror rule — A↔Z coding sends position p to 27−p. e.g. D → 27−4 = 23 → W.
- Wrap around the alphabet — After Z, a forward shift returns to A. e.g. Z + 1 → A.
The Edge
Write the alphabet with numbers once on your rough sheet at the start of the section. Every coding question after that becomes simple addition or subtraction instead of finger-counting. If a shift pushes you past Z, wrap around to A (Z + 1 = A). Forgetting to wrap is the single most common slip.Worked example
If CAT is coded as DBU, how is DOG coded?
- Find the rule from the example by comparing positions: C→D, A→B, T→U — every letter moves forward by one place.
- Confirm it is consistent across all three letters: yes, it is a uniform +1 shift.
- Apply the same +1 to DOG: D→E, O→P, G→H.
- So DOG is coded EPH.
Answer: DOG → EPH
Worked example
If in a code A = Z and B = Y, what does C become?
- A maps to Z and B maps to Y, so each letter is replaced by its mirror image in the alphabet — this is reverse coding.
- Express the rule in numbers: a letter at position p goes to position 27 − p (since 1→26, 2→25, and so on).
- C is at position 3, so it maps to position 27 − 3 = 24.
- Position 24 is the letter X.
Answer: C → X
Worked Drills
Worked example
If TEACHER is coded VGCEJGT, then CHILDREN is coded: (a) EJKNFTGP b) EJKNFGTP c) EJKMFTGP d) EJLNFTGP)
- Compare TEACHER → VGCEJGT: every letter shifts +2.
- Apply +2 to CHILDREN: C→E, H→J, I→K, L→N, D→F, R→T, E→G, N→P.
- That gives EJKNFTGP.
Answer: EJKNFTGP — option a)
Worked example
If A=1, B=2 …, and CAT = 24, then DOG equals: (a) 24 b) 26 c) 27 d) 30)
- Add the position values of DOG.
- D=4, O=15, G=7.
- 4 + 15 + 7 = 26.
Answer: 26 — option b)
Worked example
In a mirror code (A=Z), WORK is written as: (a) DLIP b) DLPI c) DKIP d) ELIP)
- Apply 27−p to each letter.
- W(23)→D(4), O(15)→L(12), R(18)→I(9), K(11)→P(16).
- That spells DLIP.
Answer: DLIP — option a)
Worked example
If + means ×, × means ÷, / means −, and − means +, then 6 + 2 × 3 / 12 − 4 equals: (a) -4 b) 4 c) 8 d) -8)
- Substitute the real operators: 6 × 2 ÷ 3 − 12 + 4.
- 6 × 2 ÷ 3 = 4.
- 4 − 12 + 4 = −4.
Answer: −4 — option a)
Worked example
If A=2, B=4, C=6 … (position × 2), then the sum of the codes of B, E, D is: (a) 20 b) 22 c) 24 d) 26)
- Each code is position × 2: B=2×2=4, E=5×2=10, D=4×2=8.
- Add them: 4 + 10 + 8.
- Total = 22.
Answer: 22 — option b)
Worked example
If ROSE is coded URVH (+3), then GARDEN is coded: (a) JDUGHQ b) JDUGHP c) JDUFHQ d) IDUGHQ)
- The rule is a +3 shift.
- Apply +3 to GARDEN: G→J, A→D, R→U, D→G, E→H, N→Q.
- That gives JDUGHQ.
Answer: JDUGHQ — option a)
Worked example
If LION → MJPO, then TIGER → (a) UJHFS b) UIHFS c) UJHGS d) TJHFS)
- LION → MJPO is a +1 shift on each letter.
- Apply +1 to TIGER: T→U, I→J, G→H, E→F, R→S.
- That gives UJHFS.
Answer: UJHFS — option a)
Worked example
If 'BAD' is written using positions as 214, then 'CAB' is: (a) 312 b) 321 c) 313 d) 231)
- Replace each letter by its position number.
- C=3, A=1, B=2.
- So CAB is 312.
Answer: 312 — option a)
⚠ Watch out
- Always confirm the shift on every letter of the example, not just the first.
- Remember to wrap around Z back to A.
- Don't confuse a forward shift with a backward one — check direction.
Takeaways
- The example pair is the whole key — decode the rule before touching the target.
- Verify the shift across every letter, never just the first.
- Mirror coding is simply p → 27−p.
- Wrap past Z back to A; that slip costs the most marks.