8.9 Coding–decoding notes

1. 8.9.1Letter coding: shifting, reversal and position values

   In letter coding every letter of a word is changed by one rule (a shift, an opposite, a reversal of order, or a mix); write the word and its code one above the other, find the rule letter by letter, and apply it.

   Standard · 7 min · Full course
2. 8.9.2Number, substitution and sentence (word-to-code) coding

   Number codes turn letters into values (positions, sums, products), substitution codes rename objects, and sentence codes hide each word behind a code word that you isolate by comparing sentences that share words.

   Standard · 8 min · Full course
3. 8.9.3Symbol and mathematical-operation coding

   Symbol coding replaces signs, numbers or relations with other symbols; rewrite the expression in ordinary signs first, then evaluate it with BODMAS.

   Standard · 7 min · Full course
4. 8.9.4Input–output (machine) coding

   A word–number machine changes a line of elements one step at a time by a fixed rule; find the rule from the worked example (which element moves, where it goes, what happens to the rest), then rerun the machine on the new input on paper.

   Standard · 8 min · Full course
5. 8.9.5Matrix and grid codes: row–column letter codes

   In a grid code each letter is named by where it sits (row, then column, unless the question says otherwise), so build or copy the grid, read off two numbers per letter, and check every pair of an option, not just the first.

   Standard · 7 min · Full course

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