Q1 If TEACHER is coded as VGCEJGT, how is CHILD coded?
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EJKNF.
Compare letter by letter: T→V, E→G, A→C. Every letter moves forward two places. Applying that to CHILD: C→E, H→J, I→K, L→N, D→F, giving EJKNF.
Coding and Decoding
What it is
Almost every coding question is one of four things: a fixed letter shift, a reversal, a rearrangement of positions, or a substitution where whole words stand for other words. Identify which before trying to decode anything.
Writing the alphabet across the top of your rough sheet with positions 1 to 26 underneath costs ten seconds and saves a minute of counting on fingers.
What you need to know
- Letter positions: A = 1 to Z = 26. Learn the anchors - E = 5, J = 10, O = 15, T = 20, Y = 25.
- The opposite letter is 27 minus the position: A ↔ Z, B ↔ Y, M ↔ N.
- A fixed shift gives the same gap for every letter. Check two or three before committing.
- If the gaps differ, look for a pattern in the gaps themselves - +1, +2, +3 is common.
- In substitution coding, nothing is a letter puzzle: build a dictionary of which word stands for which.
Worked examples
Try each one before opening the answer. A solution you have read is not a solution you can produce under time pressure, which is the only kind that counts.
Q2 In a code, 253 means "books are old", 546 means "man is old" and 378 means "buy good books". What stands for "books"?
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3.
The word "books" appears in the first and third phrases. Their digits are {2,5,3} and {3,7,8}, and the only digit common to both is 3. No other word appears in both phrases, so 3 is "books".
Q3 If in a certain language MADRAS is coded as NBESBT, how is BOMBAY coded?
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CPNCBZ.
M→N, A→B, D→E: every letter moves one place forward. So B→C, O→P, M→N, B→C, A→B, Y→Z, giving CPNCBZ. Note Y wraps to Z, not to A - the shift is only one.
Where people lose marks
- Deciding the shift from a single letter. Check at least three before applying it.
- Forgetting the alphabet wraps: Z shifted forward by one is A.
- Treating a substitution code as a letter code. If the "code" is other words, build the dictionary instead.