Coding‑Decoding: Easy Study Notes
(RPF SI – Reasoning Ability)
1. What is Coding‑Decoding?
Coding = turning a set of items (words, numbers, symbols) into a new set according to a hidden rule.
Decoding = figuring out that rule and applying it to find the missing item.
Think of it like a secret recipe: you mix ingredients in a certain order, and you must guess the order the chef used.
2. Everyday Analogy
| Real‑world | Coding | Decoding |
|---|---|---|
| Writing a note in a secret language with a key | Replace each letter with the one 2 steps ahead (C→E, D→F…) | Find the key, then read the note back to normal |
3. How to Solve a Coding‑Decoding Question
| Step | What to Do | Quick Tip |
|---|---|---|
| 1. List the items | Write the given items side‑by‑side. | Use a table or a simple list. |
| 2. Look for patterns | Check if numbers go up/down, letters shift, shapes change, etc. | Try differences, repeats, or alphabetical order. |
| 3. Test a rule | Pick a rule you think fits (e.g., “next letter is +2”). Apply it to one pair. | If it works for most pairs, keep it. |
| 4. Verify | Apply the rule to all pairs. If any pair doesn’t fit, revise your rule. | Keep trying until all pairs match. |
| 5. Decode the missing part | Use the rule on the last given item to find the missing one. | Check that the answer is a valid option (if multiple choice). |
4. Tiny Example
| A | B |
|---|---|
| 3 | 6 |
| 4 | 8 |
| 5 | ? |
Step 1: Notice 6 = 3×2, 8 = 4×2.
Step 2: Rule = “multiply by 2.”
Step 3: 5×2 = 10.
Answer: 10
5. Common Coding‑Decoding Patterns
| Pattern | How it looks | Quick Check |
|---|---|---|
| Number addition/subtraction | 5 → 8 (add 3) | Add/subtract the same number each time. |
| Number multiplication/division | 2 → 4 → 8 | Multiply by a constant. |
| Alphabet shift | A → C → E | Each letter moves forward by the same number. |
| Reverse order | 1, 2, 3 → 3, 2, 1 | The second list is the reverse of the first. |
| Skip pattern | 1, 3, 5 → 2, 4, 6 | Add 1 then add 2 alternately. |
6. Quick Tips
- Write it out – Seeing all items together helps patterns pop.
- Check one pair first – If a rule fits the first pair, test it on the rest.
- Don’t overthink – Most questions use a single simple rule.
- Use the options – If it’s multiple choice, see which option fits the rule quickly.
- Practice – The more patterns you see, the faster you’ll spot them.
7. Mini Practice Question
| X | Y |
|---|---|
| 7 | 4 |
| 8 | 5 |
| 9 | ? |
Apply the steps:
- 7 → 4 (subtract 3)
- 8 → 5 (subtract 3)
- Rule: “subtract 3.”
- 9 – 3 = 6
Answer: 6
8. Summary
- Coding‑decoding is about finding a hidden rule that turns one set into another.
- Look for simple patterns (add, multiply, shift, reverse).
- Test a rule on one pair, then apply it to all.
- Once the rule is clear, use it to find the missing item.
With these steps and a few practice questions, you’ll crack coding‑decoding problems quickly and confidently. Happy practicing!