60-second revision
- First compare the word and its code letter by letter and write the shift (+1, −2, opposite...) under each pair.
- Opposite letters add to 27 (A–Z, F–U, M–N).
- If the code looks like the word backwards, reverse first, then look for a shift.
- Number codes are usually the sum, product or reverse positions (27 − p) of letters.
- Substitution codes: answer with the code name of the thing, not the thing itself.
- Sentence codes: a word common to two sentences has the code common to the same two sentences.
- Letter–digit codes: build a two-column key from the examples, then read the new word off it.
What coding–decoding tests
A word, number or sentence is converted into a code by a fixed rule. You must find the rule from an example and apply it to a new word, or decode a given code. The rule is almost always one of a few families. Spot the family first and the rest is mechanical.
Family 1: letter shifts
Write the positions of the word and its code and subtract. The shift may be constant (+2 for every letter), increasing (+1, +2, +3...), alternating (+1, −1, +1, −1) or tied to the letter position in the word.
| Word → Code | Shifts | Rule |
|---|---|---|
| LAMP → NCOR | +2, +2, +2, +2 | constant +2 |
| WIND → XKQH | +1, +2, +3, +4 | increasing |
| ROAD → SNBC | +1, −1, +1, −1 | alternating |
| CUP → BTO | −1, −1, −1 | constant −1 |
Family 2: opposite letters
Each letter becomes the letter at the same distance from the other end: positions add to 27. A quick check: if the code of an early-alphabet letter is a late-alphabet letter (B→Y, D→W), test the opposite rule first. Anchors: A–Z, E–V, J–Q, M–N, O–L, T–G.
Family 3: reversal, with or without a shift
If the code contains the same letters as the word in reverse, it is a pure reversal (NOTE → ETON). If the letters are close to the reversed word, reverse first and then find the shift. Example: LION → OPJM? Reverse LION to NOIL, then +1 gives OPJM. Sometimes only halves are reversed: for even-length words, split in the middle and reverse each half (PLANET → ALP + TEN).
Family 4: number values
| Rule | Example | Check |
|---|---|---|
| Sum of positions | BAG = 2 + 1 + 7 = 10 | small totals |
| Product of positions | BAG = 2 × 1 × 7 = 14 | test if sum fails |
| Reverse positions (27 − p) | BAG = 25-26-20 | large values for early letters |
| Sum × number of letters | BAG = 10 × 3 = 30 | totals are multiples of word length |
| Square of positions | BAG = 4-1-49 | perfect squares |
With one example, both "sum" and "product" can fit by chance, so test both: if ADD = 9, the sum is 1 + 4 + 4 = 9 and the product is 16, so the rule is the sum.
Family 5: substitution
Words are renamed in a chain: "if chair is called table, table is called bed, bed is called sofa...". The question asks for the real-world object in code. Identify the real answer first (we sleep on a bed), then give its code name (bed is called sofa, so the answer is sofa).
Family 6: sentence (common-word) codes
Several sentences are coded word by word, but code order may not match word order. Compare two sentences: the words they share must have the codes they share. Example: "go home now" = "la mi ko", "come home soon" = "pe ko ri", "go and come" = "ri sa la". Home is common to sentences 1 and 2; ko is the common code, so home = ko. "Go" is common to 1 and 3 with code la; "come" is common to 2 and 3 with code ri.
Family 7: letter–digit keys
When words are coded as digit strings (letter for letter), build a key table from the examples. A letter that repeats in the word repeats in the code, which is a quick consistency check. Example: if SALT = 4378 and LAST = 7348, the key is S4, A3, L7, T8, so SLAT = 4738.
Decoding routine
- Count letters: if the code has the same length, it is letter-for-letter.
- Check reversal first, then shifts, then opposites.
- For numbers, try sum, product and reverse positions.
- Apply the rule to the new word and recheck one letter before marking.
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