Data Sufficiency (DS) Section

Data Sufficiency (DS) is either a part of DI or the Quant section of CAT

WHAT IS DATA SUFFICIENCY?

Every Data Sufficiency problem consists of a question followed by two statements. You have to decide NOT WHAT THE ANSWER IS, BUT WHETHER THE QUESTION CAN BE ANSWERED based on the information given in the two statements. Let us take a very simple example;

What is x?
A. x + y = 17
B. 4x + 4 = 18

As in any other question in CAT, there are five alternative answer choices.

1. if the question can be answered by statement A alone, but cannot be answered using statement B alone.
2. if the question can be answered by statement B alone, but cannot be answered using statement A alone.
3. if the question can be answered by using both the statements together, but cannot be answered by using either statement alone.
4. if the question can be answered by using either statement alone.
5. if the question cannot be answered even by using both statements together.

Here’s how to crack it

The best way to answer Data Sufficiency problems is to look at one statement at a time. So cover statement (B) with your hand. 

Based on statement A, x + y = 17 can we answer the question “What is x?” 

No way. If x + y = 17, there are many possible values of x: If y equals 2, then x could equal 15. On the other hand, if y equals 13, then x could equal 4. 

Statement A is not sufficient to give us a single value for x.

Now forget you ever saw statement A. Cover it with your finger and look only at statement B.

4x + 4 = 18. 

Based on statement B alone, can we find a single value for x?

Yes. Using algebraic manipulation, we can do the following:

4x + 4 = 18
4x = 14 
So, x = 3.5

Note that the question isn’t asking us what x equals.
The question asks only whether the information provided is sufficient to find out what x equals. In this case it is; statement B gives us enough information to answer the question “What is x?” Since B is sufficient and A is not, the answer to this question is Choice (2).

i.e., the question can be answered by one of the statements alone, but cannot be answered by using the other statement alone.

Note

CAT has been using different versions of what the four or five answer choices stand for. Therefore, it is imperative that you read the instructions given before the Data Sufficiency question in every paper and then answer them. For intance, in CAT 2003 re-test, the choices were different from what are given in the above sample. Many students who did not read the instructions carefully answered these questions wrong despite knowing how to solve them.