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Question

Given below are two statement, followed by two conclusion, I and II. You have to consider the statement to be true even if they seem to be at variance from commonly known facts. You have to decide which of the given conclusions, if any, follow(s) from the given statements.

Statement 1: No quadrilaterals are polygons.

Statement 2: All polygons are rhombuses.

Conclusion I: Some rhombuses are quadrilaterals.

Conclusion II: Some rhombuses are polygons.

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is

Only conclusion II follows

Let's carefully analyze the given statements and conclusions in this syllogism problem involving geometric shapes like quadrilaterals, polygons, and rhombuses.

Understanding the Syllogism Statements

We are given two statements:

  • Statement 1: No quadrilaterals are polygons.
  • Statement 2: All polygons are rhombuses.

We must assume these statements are true, even if they contradict commonly known facts about geometry.

In terms of set relationships:

  • Statement 1 means that the set of Quadrilaterals (Q) and the set of Polygons (P) have no elements in common. Their intersection is empty: \(Q \cap P = \emptyset\).
  • Statement 2 means that the set of Polygons (P) is entirely contained within the set of Rhombuses (R). The set of Polygons is a subset of the set of Rhombuses: \(P \subseteq R\).

Analyzing the Conclusions

We need to determine which of the following conclusions logically follow from the given statements:

  • Conclusion I: Some rhombuses are quadrilaterals.
  • Conclusion II: Some rhombuses are polygons.

Evaluating Conclusion I: Some rhombuses are quadrilaterals.

This conclusion claims that there is at least one element that is both a rhombus and a quadrilateral, meaning the intersection of Rhombuses (R) and Quadrilaterals (Q) is not empty: \(R \cap Q \neq \emptyset\).

From the statements, we know:

  • No Q are P (\(Q \cap P = \emptyset\)).
  • All P are R (\(P \subseteq R\)).

Since all polygons are rhombuses, the set of polygons is a part of the set of rhombuses. Statement 1 tells us that quadrilaterals have nothing in common with polygons.

Consider the possibility that the set of rhombuses (R) is exactly the same as the set of polygons (P). In this case, Statement 2 (All polygons are rhombuses) would be true. Statement 1 (No quadrilaterals are polygons) would also be true. Now, check Conclusion I: "Some rhombuses are quadrilaterals". If R = P, this means "Some polygons are quadrilaterals". But Statement 1 says "No quadrilaterals are polygons", which implies "No polygons are quadrilaterals" and therefore "Some polygons are quadrilaterals" is false.

Since we found a scenario where the statements are true but Conclusion I is false (R = P is a possible scenario under Statement 2), Conclusion I does not necessarily follow from the statements. The statements do not guarantee an overlap between rhombuses and quadrilaterals.

Evaluating Conclusion II: Some rhombuses are polygons.

This conclusion claims that there is at least one element that is both a rhombus and a polygon, meaning the intersection of Rhombuses (R) and Polygons (P) is not empty: \(R \cap P \neq \emptyset\).

Statement 2 says: "All polygons are rhombuses" (\(P \subseteq R\)). This means every element in the set of polygons is also in the set of rhombuses.

In standard syllogistic logic, a universal affirmative statement like "All P are R" implies the existence of P unless explicitly stated otherwise. If there are any polygons (if the set P is not empty), then because all polygons are rhombuses, those polygons are also rhombuses.

Therefore, if the set of polygons is non-empty, there exist elements that are both polygons and rhombuses. This makes the conclusion "Some rhombuses are polygons" (\(R \cap P \neq \emptyset\)) true.

Since the standard interpretation assumes the subject of a universal statement exists, Statement 2 implies the existence of polygons, and thus implies that "Some rhombuses are polygons" follows.

Conclusion Summary

  • Conclusion I (Some rhombuses are quadrilaterals) does not necessarily follow from the statements.
  • Conclusion II (Some rhombuses are polygons) follows from the statements.

Therefore, only Conclusion II follows.

Let's look at the options:

  1. Neither I nor II follows
  2. Only conclusion II follows
  3. Only conclusion I follows
  4. Both I and II follow

Based on our analysis, the correct option is the one stating that only conclusion II follows.

Revision Table: Statements and Conclusions

Item Description Set Notation
Statement 1 No quadrilaterals are polygons. \(Q \cap P = \emptyset\)
Statement 2 All polygons are rhombuses. \(P \subseteq R\)
Conclusion I Some rhombuses are quadrilaterals. \(R \cap Q \neq \emptyset\)
Conclusion II Some rhombuses are polygons. \(R \cap P \neq \emptyset\)

Additional Information on Syllogisms and Logic

This question uses categorical syllogisms, a type of logical argument that applies deductive reasoning to arrive at a conclusion based on two or more propositions that are asserted or assumed to be true. The statements and conclusions relate categories or classes of things.

  • Statements (Premises): These are the given propositions assumed to be true.
  • Conclusion: This is the proposition that is claimed to follow logically from the premises.
  • Validity: An argument is valid if the conclusion must be true whenever the premises are true. We are testing for validity here.

Types of Categorical Propositions:

  • A (Universal Affirmative): All S are P (e.g., All polygons are rhombuses) - Subject is distributed, Predicate is undistributed.
  • E (Universal Negative): No S are P (e.g., No quadrilaterals are polygons) - Both Subject and Predicate are distributed.
  • I (Particular Affirmative): Some S are P (e.g., Some rhombuses are quadrilaterals) - Neither Subject nor Predicate is distributed.
  • O (Particular Negative): Some S are not P (e.g., Some rhombuses are not polygons) - Subject is undistributed, Predicate is distributed.

In our analysis, Statement 2 (All polygons are rhombuses) is an A-type proposition. Conclusion II (Some rhombuses are polygons) is an I-type proposition. From "All P are R" (A-type), we can logically infer "Some P are R" (I-type) by subalternation (assuming P is not empty). The converse of "Some P are R" is "Some R are P", which is Conclusion II. This is a valid inference.

Statement 1 (No quadrilaterals are polygons) is an E-type proposition. Combined with Statement 2 (All polygons are rhombuses), we saw that Conclusion I (Some rhombuses are quadrilaterals) does not necessarily follow. In fact, the valid conclusion from Statement 1 and 2 is "No quadrilaterals are rhombuses" (E-type in the 4th figure, using P as the middle term: No Q are P, All P are R -> No Q are R, which is equivalent to No R are Q). If "No R are Q" is true, then "Some R are Q" must be false.

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    II. Some dolls are rings.
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