LSAT India – Logical Reasoning: Series
Series questions test your ability to determine the relative order of items given a set of constraints.
They often involve 3–6 items (people, objects, events, etc.) and may require you to deduce a full ordering or answer a specific “which of the following could be true” question.
1. Quick Definition
- Series: A set of items that must be arranged in a particular order (e.g., from first to last, earliest to latest, etc.) based on a list of conditions.
- Goal: Find the only possible sequence (or one of the few possible sequences) that satisfies all constraints.
2. Types of Series Questions
| Type | Typical Question | Example |
|---|---|---|
| Full‑order | “Which of the following is the order of the items from first to last?” | A, B, C, D, E |
| Partial‑order | “Which of the following could be true about the order?” | “C is third.” |
| Positions | “Where must a particular item appear?” | “E must be last.” |
| Adjacency | “Which items must appear next to each other?” | “B and C must be adjacent.” |
3. Strategy Checklist
| Step | What to Do | Why |
|---|---|---|
| 1. List the items | Write them in a column or row. | Keeps track of all variables. |
| 2. Translate constraints | Convert each sentence into a simple rule (e.g., “B cannot be before A” → A before B). | Clear rules are easier to apply. |
| 3. Identify fixed positions | If a rule says “X is first” or “Y is last,” place them immediately. | Locks down anchors. |
| 4. Use relative ordering | Apply “before/after” rules to create a partial ordering graph. | Builds the skeleton of the sequence. |
| 5. Check for adjacency | If two items must be next to each other, group them temporarily. | Prevents later contradictions. |
| 6. Fill remaining slots | Use the remaining items and rules; test each possibility. | Gets you to a full sequence. |
| 7. Verify all constraints | Cross‑check every rule against the final order. | Ensures no hidden mistakes. |
| 8. Answer the question | Pick the answer choice that matches the only valid sequence (or that could be true). | The final step. |
4. Key Points to Remember
| Point | Why It Matters |
|---|---|
| Order matters, not just adjacency | A rule “X is before Y” applies even if there are items in between. |
| Transitivity | If A before B and B before C, then A before C. Use this to deduce indirect relationships. |
| “Not” constraints | “X is not first” does not mean X is second; it could be third or later. |
| Multiple solutions | Some series questions allow more than one valid order. Look for the only one that satisfies all conditions, or the most restrictive choice. |
| Logical elimination | When a choice leads to a contradiction, eliminate it immediately. |
| Time management | Spend ~3–4 minutes per series question; the rest of the test is often easier. |
5. Worked Example
Question
A series of five books, numbered 1–5 from left to right, is arranged on a shelf.
- The Mystery book cannot be next to the Romance book.
- The Science book is to the right of the History book.
- The Romance book is to the left of the Science book.
- The History book is not the first book.
- The Mystery book is not the last book.
Which of the following could be the correct order from left to right?
A. History – Mystery – Romance – Science – Thriller
B. Romance – Mystery – History – Thriller – Science
C. Thriller – History – Romance – Mystery – Science
D. History – Romance – Thriller – Science – Mystery
E. Mystery – Thriller – Romance – History – Science
(Assume “Thriller” is the remaining genre.)
Step‑by‑Step Solution
- List items: History (H), Mystery (M), Romance (R), Science (S), Thriller (T).
- Translate constraints:
- M not next to R.
- S right of H (S > H).
- R left of S (R < S).
- H not first.
- M not last.
- Check each answer:
| Choice | Order | Test Constraints |
|---|---|---|
| A | H – M – R – S – T | M next to R (violation). |
| B | R – M – H – T – S | M next to R (violation). |
| C | T – H – R – M – S | M right of S? No, M left of S → violation (M not right of S). |
| D | H – R – T – S – M | - M not next to R? M is last, R is second → not adjacent → OK. - S right of H? Yes (S at 4, H at 1). - R left of S? Yes (2 < 4). - H not first? H is first → violation. Wait! H cannot be first. So D fails. |
| E | M – T – R – H – S | - M not next to R? M at 1, R at 3 → not adjacent → OK. - S right of H? S at 5, H at 4 → OK. - R left of S? 3 < 5 → OK. - H not first? H at 4 → OK. - M not last? M at 1 → OK. |
Only Choice E satisfies all constraints.
Answer: E
6. Quick Revision Summary
| Topic | What