Logical Reasoning is one of the toughest topis in CSAT exam. A paragraph is given and student is required to answer the questions that follow based on the information given in the paragraph. They have to use their logic to interpret the question and answer it correctly.
You must read the question and directions thoroughly before answering. You must develop the habit of reasoning based on the following guidelines.
In logic, three kinds of logical reasoning can be distinguished: deduction, induction and abduction. Given a precondition, a conclusion, and a rule that the precondition implies the conclusion, they can be explained in the following way:
- Deduction means determining the conclusion. It is using the rule and its precondition to make a conclusion. Example: "When it rains, the grass gets wet. It rains. Thus, the grass is wet."
- Induction means determining the rule. It is learning the rule after numerous examples of the conclusion following the precondition. Example: "The grass has been wet every time it has rained. Thus, when it rains, the grass gets wet."
- Abduction means determining the precondition. It is using the conclusion and the rule to support that the precondition could explain the conclusion. Example: "When it rains, the grass gets wet. The grass is wet, it must have rained."
Conditions
The first basic concept that examinees should understand is the different uses of conditions. There are two types of conditions that examinees will see in logical reasoning (arguments) problems: (1) necessary conditions and (2) sufficient conditions. It is critical to master the differences between these two types of conditions.
A necessary condition is a condition that is required before a certain result can occur. For example, air must be blown into a balloon before it will expand.
On the other hand, a sufficient condition is a condition that could be adequate, on its own, to lead to a certain result. For example, if you are trying to determine who blew up the balloon, stating that Amit has the capacity to blow up the balloon is merely a sufficient condition with respect to the result that Amit blew up the balloon. This is because Amit is presumably one of many that could have blown up the balloon, but that does not mean that he did. It would have to be shown that only Amit could have blown up the balloon for the condition to be necessary.
Examples:
1. Whenever A sings, B gets a headache and C complains. If C is not complaining, which of the following statements must be true?
(A) A is singing and B has a headache.
(B) B has a headache but A is not necessarily singing.
(C) A is singing, but B does not necessarily have a headache.
(D) A has been singing and B is beginning to get a headache.
(E) A is not singing.
Solution: Since if A sings, then B gets a headache and C complains. Then we're told that C is not complaining. Using the contrapositive, we get the following logic: If B doesn't have a headache and/or if C isn't complaining, then A must not be singing. So if C is not complaining, then there is no way that A is singing, and choice (E) is the answer.
2. A will eat the apple if B does not cook.
Based only on the information above, which of the following must be true?
(A) A will not eat the apple if B cooks.
(B) If A did not eat the apple, B did cook.
(C) If A eats the apple, then B did not cook.
(D) If B does not cook, A will not eat the apple.
(E) If A did not eat the apple, B did not cook.
Solution: The sentence can be rearranged to read: "If B does not cook, then A will eat the apple. Whenever a Logical Reasoning question gives you an "If...then"sentence, the only thing that must be true is called the "contrapositive"—take the opposite of each half of the statement, and flip the two halves. For this question, the contrapositive is "If A did not eat the apple, then B did cook." Choice (B) matches this perfectly.
