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Various do's and don'ts of Data Sufficiency

The process to solve the DS questions is a bit different from the process involved in solving the QA or LR/DI questions. There are certain key things that we should keep in mind before solving the DS questions. Let us look at them one by one:

Understanding the options

This is the first step in solving the DS questions. One must go through the options and understand them clearly. Sometimes, the options given in data set themselves can become a perennial source of problem.

Let us understand the statements given in the options:
 
If the option statement reads like — “If the question can be solved by using only one of the statements alone, but not by the other statement alone”.
 
It means—Out of the given two (or, more) statements, only one statement can answer the question given, and the other statement cannot give the answer to the question given. In this case either the first statement will give the answer or the second one, but not both simultaneously i.e., if the first statement can give the answer, then second statement cannot give the answer and vice-versa.
 
If the option statement reads like— “If the question can be solved by using either of the statements alone”.
 
It means—Out of the given two (or more) statements, the question can be solved by using any one of the statements alone. In this case either first statement will give the answer or the second one used individually and independently.
 
If the option statement reads like — “If the question can be solved by using both (or all) the statements together, but not by either of the statements alone”.
 
It means—Out of the given two (or more) statements, the question can be solved only if the information given in both (or all) the statements is used together. And, using only one of the statements alone cannot solve the given question.
 
If the statement reads like— “If the question cannot be solved by even using both the statements together”.
 
It means—Out of the given two (or more) statements, the question cannot be solved even by using the information given in both (or all) the statements together.
 
Once you have understood the statements given in the options, you must not forget to see which statement corresponds to which option. CAT is known to introduce the surprise element quite often, and this also can be one of those surprises which might catch you unaware if you are not careful about this basic fact.

General awareness/assumptions are not allowed

As an unproclaimed rule, the element of general awareness cannot be used to solve the DS questions. However, this is also true for QA and LR /OI, but its importance in DS increases manifold.

Let us understand this with an example:
 
Example
Is New Delhi the most polluted city in India?
 
Statement 1 – The Capital of India is the most polluted city in India.
 
Statement 2 – New Delhi is the capital.
 
Solution
Well, we can understand that statement 1 alone cannot answer the question, as it is not given that New Delhi is the capital of India.
 
If your answer is “Both the statements together can answer the question”, think again. It is not given in statement 2 that New Delhi is the capital of India.
 
So, answer to this question is “Cannot be determined.”
 
However, we should have a clear idea regarding the differences between a universal fact like “Pythagoras theorem” and general awareness like “New Delhi is the capital of India”.
 
Universal facts like mathematical principles are not the general awareness stuff, and hence can be used.
 
 
Example
In how many days A and B working together can finish a work?
 
Statement I— A can finish the same work alone in 10 days.
 
Statement 2— B can finish the same work alone in 15 days.
 
Solution
Understandably, we cannot have a unique answer to this question by using the statements alone. However, in this case we cannot solve this question even by using both the statement together.
 
 
Flaw Detector—Had this question been a question in Quantitative Aptitude, we must have got the answer by using the methods of time and work. However, in this case of data sufficiency, since it is not given that A and B start working together and kept on working together till the work is finished, we cannot solve this question even by using both the statements together. And we cannot make assumptions that they started working together and kept on working till the work is finished.





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