Suppose that in a particular code, TIGER is written as QDFHS and MERIT is written as SHQDL, then in the same code, FROZEN will be written as -

Correct — option (3), MDYNQE.

A clue to the rule: T, the first letter of TIGER, sits next to S, the last letter of QDFHS, in the alphabet. That points to a reversed order plus a shift.

Step 1 — find the rule from TIGER → QDFHS.
Write TIGER backwards: R E G I T
Move each letter one place back: R→Q, E→D, G→F, I→H, T→S
Result: QDFHS, which matches

Step 2 — test the rule on MERIT → SHQDL.
Backwards: T I R E M
One place back: S H Q D L
Result: SHQDL, which matches

Step 3 — apply the rule to FROZEN.
Backwards: N E Z O R F
One place back: N→M, E→D, Z→Y, O→N, R→Q, F→E
Result: MDYNQE

The idea to remember: settle the order of the letters and the shift of each letter as two separate steps, and confirm the rule on both examples.

Why the others are wrong

• (1)
  MEYNQD — MEYNQD matches the rule in four places but swaps the 2nd and 6th letters.

  The 2nd letter comes from E, the 2nd letter of NEZORF, moved back to D. The 6th comes from F, moved back to E. MEYNQD puts E second and D last.

• (2)
  MDEZOR — MDEZOR matches the rule only in its first two letters, M and D, from N and E.

  The rule turns the next four letters of NEZORF, which are Z, O, R and F, into Y, N, Q and E. MDEZOR has E, Z, O and R there, letters of FROZEN that have not been shifted.

• (4)
  EQNYDM — EQNYDM is FROZEN shifted one place back without being reversed: F→E, R→Q, O→N, Z→Y, E→D, N→M.

  It is MDYNQE written backwards. The TIGER and MERIT examples show that the code reverses the order as well as shifting each letter.

Letter codes are built from a few operations: shifting each letter a fixed number of places, reversing the order, swapping letters in pairs or blocks, or replacing letters by their positions.

When the code uses letters close to those of the word but in a different order, separate the two operations. Find the order first by matching letters that sit next to each other in the alphabet, then read the shift.

A rule is confirmed only when it turns every given word into its given code.

Alphabet positions help with shifts: A = 1, E = 5, N = 14, O = 15, R = 18, Z = 26.

Key facts

• In this code, the word is written backwards and each letter moves one place back in the alphabet.
• TIGER → REGIT → QDFHS, and MERIT → TIREM → SHQDL.
• FROZEN → NEZORF → MDYNQE.
• Shifting FROZEN one place back without reversing gives EQNYDM, the answer's letters in reverse order.
• Alphabet positions: E 5, F 6, N 14, O 15, R 18, Z 26.

Common traps

• Shifting without reversing. FROZEN moved one place back is EQNYDM, the answer's letters in reverse order.
• Shifting forward instead of back. In the first example T becomes S, one place back, not U.
• Checking only one example. The rule has to turn both TIGER into QDFHS and MERIT into SHQDL before it is applied to FROZEN.

• practice — not a real PYQ

  In a certain code, LAMP is written as QNBM. How is DESK written in that code?

  • (a)LTFE
  • (b)EFTL
  • (c)JRDC
  • (d)KSED
  Answer(1) — LAMP backwards is PMAL, and moving each letter one place forward gives QNBM. DESK backwards is KSED, which becomes LTFE.

  Option (2) shifts forward without reversing; option (3) reverses but shifts back; option (4) reverses without shifting.

• practice — not a real PYQ

  In a certain code, CAT is written as DBU and DOG as EPH. How is COW written in that code?

  • (a)DPX
  • (b)XPD
  • (c)BNV
  • (d)DQX
  Answer(1) — Each letter moves one place forward, with no reversal: C→D, O→P, W→X. Option (2) also reverses the order; option (3) moves each letter back; option (4) moves O two places instead of one.