Letter and Number Coding
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Key points
- Check for reversal before comparing letter by letter
- Constant differences mean letter shifting, with wrap-around past Z
- Pairs adding to 27 mean opposite-letter coding
- The same letters in a new order mean a position swap
- In number coding test position values, their sum and their product
- In conditional coding classify each letter as vowel or consonant first
- Code only the first two letters to eliminate most options
On this page
What coding-decoding is
Coding means converting a word, number or sentence into another form by a fixed rule. Decoding means working out that rule and recovering the original.
How to attack any coding question
- Write the original word and the code one below the other, letter by letter.
- Check whether the number of letters matches and whether the order is reversed.
- Find the position difference for each pair, using the EJOTY anchors.
- If the difference is not constant, test opposite letters or a change of position.
- Apply the same rule to the new word, and confirm it still fits the given example.
Key point
Check for reversal before comparing letter by letter. If the code is the word written backwards and you compare it forwards, no rule will ever appear.
The main coding types
| Type | Clue | Method |
|---|---|---|
| Letter shifting | The same difference between each pair | Coded = original plus or minus k, with wrap-around past Z |
| Opposite letter | Differences vary but each pair adds to 27 | Coded position = 27 minus the original position |
| Reversal | The code looks like the word read backwards | Reverse first, then check for any shift |
| Position swap | The same letters in a different order | Compare positions, not letter values |
| Number coding | The code is made of numbers | Test position values, their sum, or their product |
| Conditional coding | Vowels and consonants change differently | Classify each letter before coding it |
Worked examples
Letter shifting
OFFICE is coded as PGGJDF. Each letter moves one place forward, since O goes to P and F to G. Applying the same rule, RECORD becomes SFDPSE.
Opposite letters
FILE is coded as UROV. Here F at 6 pairs with U at 21, I at 9 with R at 18, L at 12 with O at 15, and E at 5 with V at 22. Every pair adds to 27, so NOTE becomes MLGV.
Reversal with a shift
MONEY is coded as ZFOPN. Reversing MONEY gives YENOM, and adding one to each letter gives Z, F, O, P, N. So BOARD reverses to DRAOB and becomes ESBPC.
Position swap
PLAYER is coded as LPYARE. Each pair of adjacent letters is swapped, so PL becomes LP, AY becomes YA and ER becomes RE. Therefore SENIOR becomes ESINRO.
Number coding
If CAT is 24 and DOG is 26, the code is the sum of the position values, since 3 + 1 + 20 = 24 and 4 + 15 + 7 = 26. So COW is 3 + 15 + 23 = 41.
Conditional coding
If every vowel moves one place forward and every consonant one place back, BRAIN becomes AQBJM: B to A, R to Q, A to B, I to J and N to M.
Exam tip
There is no need to code the whole word. Code the first two letters and most options fall away; then check only the letter where the survivors differ.
Practice questions
Answer all, then check. Explanations appear after checking.
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