Data Sufficiency  Most Confusing Questions In GMAT Quant
What is Data Sufficiency?
Data sufficiency is a question format seen in competitive/admission exams. It is used to assess a student’s decision making skills given few conditions/information. As the name suggests, questions in data sufficiency expect one’s analysis of the information given, whether sufficient or not. And hence make conclusions based on the same.
The GMAT has good number of the Quant questions in the data sufficiency format. These questions can be confusing as these primarily test one on whether he/she has taken all the given information into consideration while making decisions. For someone who knows nothing about data sufficiency, it would be baffling to understand its complexity and the approaches to solve.
Components in a Data Sufficiency Question
There are three components in a data sufficiency question.
The only clear thing in data sufficiency problems are the answer choices given
A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked
B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the question asked
C) BOTH statements (1) and (2) TOGETHER are sufficient to answer the question asked, but NEITHER statement ALONE is sufficient to answer the question asked
D) EACH statement ALONE is sufficient to answer the question asked
E) Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed
Example :
Lee has $5 less than the amount his friend Chan has. Chan buys 2 mobile handsets with the all the money he has. What is the price of each of the equally priced handsets Chan buys?
(I) Lee can buy only one such handset with the amount he has
(II) The price of the handset is 5 more than the least nonnegative multiple of 5
A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked
B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the question asked
C) BOTH statements (1) and (2) TOGETHER are sufficient to answer the question asked, but NEITHER statement ALONE is sufficient to answer the question asked
D) EACH statement ALONE is sufficient to answer the question asked
E) Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed
Now that we have seen the three parts of the question, let’s also understand the three parts of the solution
Generally, the first step to solve a data sufficiency questions is to take stock of the information. And to check if the statement 1 was sufficient to answer the question. In this question, the universal information given is that Lee has $5 less than the amount Chan has. And also that Chan buys 2 equally priced mobile handsets with all the money he has. The first statement tells us that Lee can buy only one such handset with the amount he has. So, if Lee has $x with him then the price of the handset would also be $x. But, we know that Chan buys 2 mobile handsets with all the money he has and hence should have $2x with him. Now, since Lee has $5 less than what Chan has, x = 2x – 5 which implies x =5. Hence, we have got the price of each of the handset using the first statement alone. Statement 1 is alone sufficient now.
The second step now is to check the sufficiency of the statement 2. The price of the handset is 5 more than the least nonnegative multiple of 5. We now have to use some knowledge about number properties, apart from the information given in the question. The least nonnegative multiple of 5 is 0 and 5 more than 0 is 5. Hence, we get the price of the handset using the statement 2 alone too. Hence Statement 2 is alone sufficient too.
Hence, the right answer for this question would be D (Each statement alone is sufficient to answer the question asked).
Now, notice that we have only done 2 steps above to solve the question. While the above 2 steps would be mandatory steps for solving any data sufficiency questions, the third step of combining the 2 statements would be required only when the right answer turns out to be C or E (when neither of the 2 statements are individually sufficient).
Let’s make a small change to the same question above to understand the third step of combining 2 statements.
Suppose, the question stem remained the same, but the 2 statements were:
(I) Chan and Lee together have $x
(II) x = 3x – 30
Now, the statements 1 & 2 above individually would not suffice to answer the question. We hence perform the third step of combining the 2 statements. The statement 2 helps get the value of x as 15. And, if we assume the price of each handset to be p, then using statement 1, we get 2p + (2p5) = x. Now, combining 2 statements, we have 2p + (2p5) = 15 which implies p=5. Hence, the 2 statements individually are not sufficient but when combined they are sufficient to answer the question. Hence, the right answer would be C.
If at all, after combining both the statements also, we find the question unanswerable, then we choose E as the right answer.
Top tips to ace GMAT data sufficiency
Data sufficiency is a question format seen in competitive/admission exams. It is used to assess a student’s decision making skills given few conditions/information. As the name suggests, questions in data sufficiency expect one’s analysis of the information given, whether sufficient or not. And hence make conclusions based on the same.
The GMAT has good number of the Quant questions in the data sufficiency format. These questions can be confusing as these primarily test one on whether he/she has taken all the given information into consideration while making decisions. For someone who knows nothing about data sufficiency, it would be baffling to understand its complexity and the approaches to solve.
Components in a Data Sufficiency Question
There are three components in a data sufficiency question.

Information in the question

Data sufficiency statements

Answer Choices
The only clear thing in data sufficiency problems are the answer choices given
A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked
B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the question asked
C) BOTH statements (1) and (2) TOGETHER are sufficient to answer the question asked, but NEITHER statement ALONE is sufficient to answer the question asked
D) EACH statement ALONE is sufficient to answer the question asked
E) Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed
Example :
Lee has $5 less than the amount his friend Chan has. Chan buys 2 mobile handsets with the all the money he has. What is the price of each of the equally priced handsets Chan buys?
(I) Lee can buy only one such handset with the amount he has
(II) The price of the handset is 5 more than the least nonnegative multiple of 5
A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked
B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the question asked
C) BOTH statements (1) and (2) TOGETHER are sufficient to answer the question asked, but NEITHER statement ALONE is sufficient to answer the question asked
D) EACH statement ALONE is sufficient to answer the question asked
E) Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed
Now that we have seen the three parts of the question, let’s also understand the three parts of the solution
Generally, the first step to solve a data sufficiency questions is to take stock of the information. And to check if the statement 1 was sufficient to answer the question. In this question, the universal information given is that Lee has $5 less than the amount Chan has. And also that Chan buys 2 equally priced mobile handsets with all the money he has. The first statement tells us that Lee can buy only one such handset with the amount he has. So, if Lee has $x with him then the price of the handset would also be $x. But, we know that Chan buys 2 mobile handsets with all the money he has and hence should have $2x with him. Now, since Lee has $5 less than what Chan has, x = 2x – 5 which implies x =5. Hence, we have got the price of each of the handset using the first statement alone. Statement 1 is alone sufficient now.
The second step now is to check the sufficiency of the statement 2. The price of the handset is 5 more than the least nonnegative multiple of 5. We now have to use some knowledge about number properties, apart from the information given in the question. The least nonnegative multiple of 5 is 0 and 5 more than 0 is 5. Hence, we get the price of the handset using the statement 2 alone too. Hence Statement 2 is alone sufficient too.
Hence, the right answer for this question would be D (Each statement alone is sufficient to answer the question asked).
Now, notice that we have only done 2 steps above to solve the question. While the above 2 steps would be mandatory steps for solving any data sufficiency questions, the third step of combining the 2 statements would be required only when the right answer turns out to be C or E (when neither of the 2 statements are individually sufficient).
Let’s make a small change to the same question above to understand the third step of combining 2 statements.
Suppose, the question stem remained the same, but the 2 statements were:
(I) Chan and Lee together have $x
(II) x = 3x – 30
Now, the statements 1 & 2 above individually would not suffice to answer the question. We hence perform the third step of combining the 2 statements. The statement 2 helps get the value of x as 15. And, if we assume the price of each handset to be p, then using statement 1, we get 2p + (2p5) = x. Now, combining 2 statements, we have 2p + (2p5) = 15 which implies p=5. Hence, the 2 statements individually are not sufficient but when combined they are sufficient to answer the question. Hence, the right answer would be C.
If at all, after combining both the statements also, we find the question unanswerable, then we choose E as the right answer.
Top tips to ace GMAT data sufficiency

Always remember the answer choices

Consider the statements individually first

Be Clear on what makes a statement sufficient/insufficient

Eliminate incorrect choices

Picking numbers as a reflex

Considering that all the statements give new information

Don’t look at the statements together first without looking them individually in general

Don’t solve the question completely. Check if the statements are sufficient or not