Coding-decoding and series: stop counting the alphabet by hand

Coding-decoding is among the most reliable marks in the reasoning section, and almost the only way to lose them is by counting the alphabet on your fingers. Fix the position anchors and learn to name the coding type on sight, and these questions collapse to a few seconds each.

Anchor the alphabet once

Two facts remove nearly all the counting:

• EJOTY. The letters E, J, O, T, Y sit at positions 5, 10, 15, 20, 25. Any letter is a short hop from the nearest anchor — S is just before T(20), so 19.
• The 27 rule. A letter and its mirror from the far end sum to 27. So the opposite of G (7) is the 20th letter, T; the opposite of D (4) is W (23). This is what makes “reverse the alphabet” coding instant.

Write these on your rough sheet at the start of the section, and never count from A again.

Name the coding type, then apply the tool

• Letter-shift coding. Each letter moves a fixed number of places: CAT → DBU is a +1 shift. Find the shift on one letter, apply to all. Watch for shifts that go backwards or that differ by position.
• Number coding. Letters map to numbers — sometimes their position, sometimes position squared, sometimes a sum. Test the simplest rule first.
• Substitution coding. Whole words are swapped: “sky is blue” is written with three code words. Only the mapping matters; there is no letter logic.
• Decipher (message) coding. You are given several coded sentences with a shared word or two, and must deduce which code means which word by comparing what the sentences have in common. Line them up and match the repeated code to the repeated word.

Naming the type is 90% of the work, because it tells you whether to look at letters, positions, or whole words.

Alphanumeric and symbol series

These give a long string of letters, digits and symbols and ask a targeted question. The method is discipline, not cleverness:

• Read the question first. It is usually specific: “how many symbols are immediately preceded by a digit and followed by a letter?”
• Make one left-to-right pass applying exactly that condition, marking as you go. Do not try to absorb the whole string; scan for the pattern the question named.

Most errors here are from re-reading the string several times under time pressure. One clean, condition-focused pass is faster and more accurate.

A worked reading

If in a code L = 12 and the word is coded by adding the positions, then “ICE” is 9 + 3 + 5 = 17. With EJOTY, I is one before J(10) so 9, C is 3, E is 5 — read off in seconds, no counting from A. The setter budgeted the counting; the anchors are how you refuse to pay it.

Where the marks leak

Counting from A. Slow and error-prone; the anchors and the 27 rule exist precisely to stop this.

Forward versus backward shift. Confirm the direction on a second letter before applying the shift to the whole word.

Position versus value. Decide whether the code uses a letter’s alphabet position or some operation on it, and check the rule on two letters, not one.

Trying to read the whole series. For alphanumeric strings, scan once for the specific condition asked; do not parse the entire string.

Practice that transfers

Coding rewards recall of the anchors and instant type-recognition, so drill both. Work through the coding, decoding and series practice set, and keep the number sense underneath quick with calculation speed so position arithmetic never slows you down.