Coding and Decoding

About Coding and Decoding

In a coding question a word is shown alongside its coded form, and you are asked to code a second word using the same rule. The worked example is the whole answer: compare its letters position by position, state the rule in words and then apply it. The rule is almost always applied to each letter independently.

What you need to understand

Compare the plain word with the coded word letter by letter to find the rule.

A constant shift moves every letter the same number of places forward or backward in the alphabet.

In a position code each letter is replaced by its number in the alphabet, so A is 1 and Z is 26.

The rule stays the same for the second word; only the letters change.

If shifting forward would pass Z, the alphabet wraps around and counting continues from A.

How to work through these questions

Write the plain word and the coded word one above the other.

Compare the first pair of letters and describe what happened to it.

Check that the same description holds for every other letter of the example.

Apply the rule to each letter of the second word in turn.

Check the answer by applying the rule backwards to see whether the original word returns.

Mistakes that cost marks

Deriving the rule from one letter only, when the example contains a different rule for later letters.
Reversing the shift, so the answer is coded the wrong way.
Applying the rule to the example again instead of to the new word.
Forgetting that the alphabet wraps around from Z back to A.
Counting alphabet positions from zero, which puts every letter one place out.

Worked example

In a certain code, TABLE is written as UBCMF. Using the same code, how is CHAIR written?

Compare T with U, A with B, B with C and so on. Each letter has moved one place forward in the alphabet.
Check the last pair as well: E becomes F, one place forward, so the rule holds throughout.
Apply the rule to CHAIR by moving each letter one place forward.
C becomes D, H becomes I, A becomes B, I becomes J and R becomes S.

Answer: DIBJS

Practice questions with answers

A few Coding and Decoding questions with the full solution shown, so you can see how the method is applied before you attempt the timed set.

Question 1

In a certain code, BEACH is written as EHDFK. Using the same code, how is RKTCL written?

A UNWFO
B OHQZI
C WPYHQ
D VOXGP

Answer: Option A — with explanation

Comparing BEACH with EHDFK letter by letter, each letter moves 3 places after in the alphabet. Applying the same shift to RKTCL letter by letter gives UNWFO. Common mistakes - shifted by 5 places instead of 3: WPYHQ - shifted the letters backwards instead: OHQZI - shifted by 4 places instead of 3: VOXGP

Question 2

In a certain code, RIVER is written as 18 9 22 5 18. Using the same code, how is MFXSV written?

A 14 7 25 20 23
B 13 6 24 19 22
C 12 5 23 18 21
D 22 19 24 6 13

Answer: Option B — with explanation

Each letter is replaced by its position in the alphabet: the first letter of RIVER is 18 and the last is 18. Applying the same rule to MFXSV gives 13 6 24 19 22. Common mistakes - started the count one letter too early: 12 5 23 18 21 - counted the alphabet from zero: 14 7 25 20 23 - wrote the positions in the wrong order: 22 19 24 6 13

Question 3

In a certain code, TABLE is written as RYZJC. Using the same code, how is VDSLT written?

A RZOHP
B SAPIQ
C XFUNV
D TBQJR

Answer: Option D — with explanation

Comparing TABLE with RYZJC letter by letter, each letter moves 2 places before in the alphabet. Applying the same shift to VDSLT letter by letter gives TBQJR. Common mistakes - shifted by 3 places instead of 2: SAPIQ - shifted by 4 places instead of 2: RZOHP - shifted the letters forwards instead: XFUNV

Question 4

In a certain code, MUSIC is written as 13 21 19 9 3. Using the same code, how is VDSLT written?

A 23 5 20 13 21
B 22 4 19 12 20
C 20 12 19 4 22
D 21 3 18 11 19

Answer: Option B — with explanation

Each letter is replaced by its position in the alphabet: the first letter of MUSIC is 13 and the last is 3. Applying the same rule to VDSLT gives 22 4 19 12 20. Common mistakes - wrote the positions in the wrong order: 20 12 19 4 22 - started the count one letter too early: 21 3 18 11 19 - counted the alphabet from zero: 23 5 20 13 21

Question 5

In a certain code, LIGHT is written as PMKLX. Using the same code, how is MFXSV written?

A SLDYB
B RKCXA
C IBTOR
D QJBWZ

Answer: Option D — with explanation

Comparing LIGHT with PMKLX letter by letter, each letter moves 4 places after in the alphabet. Applying the same shift to MFXSV letter by letter gives QJBWZ. Common mistakes - shifted the letters backwards instead: IBTOR - shifted by 5 places instead of 4: RKCXA - shifted by 6 places instead of 4: SLDYB

Frequently asked questions

How do I find the code?

Line the plain word up with its coded form and compare the letters in matching positions. The change to the first letter is almost always the rule, and the rest of the example confirms it.

What if the coded form is numbers rather than letters?

That is a position code: each letter is replaced by its number in the alphabet, so A is 1 and Z is 26. Check the example to confirm the counting starts at one.

What happens at the end of the alphabet?

The letters wrap around. One place forward from Z is A, and one place back from A is Z.

Should I apply the rule to the example as a check?

Yes, but apply it backwards. Coding the plain word again only repeats what you were given, whereas decoding the coded word and recovering the original proves the rule.

Why is one of the wrong options often a plausible word?

Because the wrong options are built from specific mistakes, such as shifting the wrong way or by the wrong amount. They are designed to catch a particular error rather than to be random.