Coding-decoding — Practice MCQs

Coding-decoding is the single most dependable scoring area in the OPSC ASO reasoning paper: every paper from 2012 to 2025 has asked it, with 5 to 7 questions in each of the last three papers. Almost every question uses one of eight fixed patterns. Learn to spot the pattern from the example word in under 20 seconds, then apply it mechanically.

First, write the alphabet with positions

Before you start, write this once on your rough sheet. It saves time on every question.

• Memory trick EJOTY: E = 5, J = 10, O = 15, T = 20, Y = 25. Count from the nearest one.
• Opposite letters: two letters are opposite when their positions add to 27 (A–Z, B–Y, C–X, M–N). Pairs to remember: AZ, BY, CX, DW, EV, FU, GT, HS, IR, JQ, KP, LO, MN.
• Reverse position of a letter = 27 − position (A = 26, Z = 1).

The eight patterns OPSC uses

1. Fixed shift (+k or −k)

Every letter moves the same number of places. FRIEND → GSJFOE is +1. To check, compare only the first two letters of the word and its code. If both have moved by the same amount, test the rest.

2. Increasing or alternating shift

The shift changes in a regular way. CAT → DCW is +1, +2, +3. PLANET → QKBMFS is +1, −1, +1, −1. Write the shift above each letter and the pattern becomes clear.

3. Opposite letters

Each letter is replaced by its opposite: GLOW → TOLD. When the code looks like the "mirror" end of the alphabet, check whether the position pairs add to 27.

4. Rearrangement: reverse, pair swap, half reverse

• Full reverse: NATION → NOITAN.
• Pair swap (ASO 2025): STRAIGHT → ST RA IG HT → TS AR GI TH = TSARGITH. If the word has an odd number of letters, the last one stays in place.
• Half reverse: BRIGHTEN → BRIG | HTEN → GIRB NETH.

Tip: if the code uses the same letters as the word, it is a rearrangement. Don't look for shifts.

5. Letter → number (sum of positions)

The most common ASO pattern of recent years. Add the positions and look for a simple link.

• ASO 2025: N = 28, ORE = 76. N = 14 and ORE = 15 + 18 + 5 = 38, so the code is 2 × sum. PALE = 2 × (16 + 1 + 12 + 5) = 68.
• ASO 2025: MOST = 134 and FUR = 90 → 2 × sum. SUCCESS = 2 × 89 = 178.
• ASO 2018: Z = 52, ACT = 48 → 2 × sum. BAT = 2 × 23 = 46.
• ASO 2015: D = 4, COVER = 63 → plain sum. BASIS = 50.

Order to test the rules: plain sum → 2 × sum → sum ± a constant → sum + number of letters → reverse-position sum. Always check the rule on both examples before you apply it.

6. Letter → digit (fixed substitution)

Each letter always gets the same digit. Example (ASO 2018): ROSE = 6821, CHAIR = 73456 and PREACH = 961473. Make a small table (R = 6, O = 8, S = 2, E = 1, C = 7, H = 3, A = 4, I = 5), then SEARCH = 214673. Letter-to-letter substitution works the same way (TUTORIAL → DODNGLCF, DANCE → YCJMZ, so EDUCATION → ZYOMCDLNJ).

7. Sentence or word codes

Example: ‘first of all’ = ‘kan dan san’, ‘who is first’ = ‘zan kan ven’ and ‘this is pale’ = ‘ven gen len’.

1. Pick two sentences that share a word. The code they share belongs to that word: first → kan, is → ven.
2. Remove the known codes. The single leftover code belongs to the leftover word: who → zan.

The order of code words is usually jumbled, so never use position to decide.

8. Substitution ("cloud is called white")

Find the real answer first, then give its new name. Birds fly in the air, and air is called blue, so the answer is blue. The most common mistake is answering with the real word.

Exam strategy

• Use the options. If they differ only in one or two positions, decode just those positions.
• Coding questions are usually the fastest marks in the paper (about 30–40 seconds each), so attempt them first.
• With −0.25 negative marking, skip a sentence-code question only if the given data really can't fix the word. Usually it can.