Concept:
- A letter-shift code can be treated as ordinary arithmetic by numbering the letters $A=0, B=1, \dots, Z=25$, so the encoding rule becomes $\text{code}(x) = (x+s) \bmod 26$ for some fixed shift $s$.
- The shift $s$ can be found from the given example and should be confirmed with more than one letter pair before it is trusted.
Step 1: Convert one letter pair from the given example to numbers to find the shift.
$O$ is the 15th letter, so $O = 14$ (0-indexed). $P$ is the 16th letter, so $P = 15$.
Shift $s = 15 - 14 = 1$
Step 2: Confirm the same shift with another pair from the example.
$E = 4$, $F = 5$, so shift $= 5 - 4 = 1$, the same value. The rule $s = 1$ is confirmed, not a coincidence.
Step 3: Convert each letter of GRAPES to its 0-indexed number.
$G = 6,\ R = 17,\ A = 0,\ P = 15,\ E = 4,\ S = 18$
Step 4: Apply $\text{code}(x) = (x+1) \bmod 26$ to each value and convert back to letters.
$G: 6+1=7 \to H$
$R: 17+1=18 \to S$
$A: 0+1=1 \to B$
$P: 15+1=16 \to Q$
$E: 4+1=5 \to F$
$S: 18+1=19 \to T$
Final Answer: $HSBQFT$