Question

All hill-stations have a lake. Ooty has two lakes.

Which of the statement(s) below is/are logically valid and can be inferred from the above sentences?
(i) Ooty is not a hill-station.
(ii) No hill-station can have more than one lake.

Choose one option
Official answer and solution
Official answer
D
From
Official GATE 2016 answer key

Solution

Formalize the premises using predicate logic to verify whether either conclusion is necessarily valid.

Given premises:

  1. "All hill-stations have a lake." In predicate logic:
    ∀x(HillStation(x)  ⟹  HasLake(x))\forall x (\text{HillStation}(x) \implies \text{HasLake}(x))
    Here, having "a lake" means having at least one lake (≥1)(\ge 1). It does not restrict the number of lakes to exactly one, nor does it state that only hill-stations have lakes.
  2. "Ooty has two lakes." That is, Lakes(Ooty)=2\text{Lakes}(\text{Ooty}) = 2.

Evaluate statement (i): "Ooty is not a hill-station."
Since Ooty has 2 lakes, it satisfies the condition of having at least one lake. It is entirely possible that Ooty is a hill-station, or that it is not. The premises do not entail that Ooty is not a hill-station. Thus, (i) cannot be inferred.

Evaluate statement (ii): "No hill-station can have more than one lake."
The premise states that every hill-station has at least one lake; it sets a lower bound, not an upper bound. A hill-station can certainly have multiple lakes without violating the premise. Thus, (ii) cannot be inferred.

Therefore, neither (i) nor (ii) is logically valid.

Final answer

D

Common trap

Interpreting "have a lake" as "have exactly one lake", or committing the fallacy of denying the antecedent.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper
GATE 2016 Computer Science and Information Technology (CS) Session 2
Question
8 of 65, printed as GA Q.8
Section
General Aptitude

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