This data sufficiency problem consists of a q...
This data sufficiency problem consists of a question and two statements, labeled (1) and (2), in which certain data are given. You have to decide whether the data given in the statements are sufficient for answering the question. Using the data given in the statements, plus your knowledge of mathematics and everyday facts (such as the number of days in a leap year or the meaning of the word counterclockwise), you must indicate whether -
Q.  How many of the numbers x, y, and z are positive if each of these numbers is less than 10?
1. x + y + z = 20
2. x + y = 14
• 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)
• e)
Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed.
This data sufficiency problem consists of a question and two statement...
Step 1 of solving this GMAT DS question: Understand the Question Stem
What kind of an answer will the question fetch?
The question is a "How many?" question. For questions asking "how many", the answer should be a number.
When is the data sufficient?
The data is sufficient if we are able to get a UNIQUE answer for the number of positive numbers from the information in the statements.
If the statements do not have adequate data to uniquely determine how many among the three numbers are positive, the data is NOT sufficient.
Key data from the question stem
Each of the three numbers x, y, and z are less than 10.
Step 2 of solving this GMAT DS question:
Evaluate Statement (1) ALONE: x + y + z = 20
From the question stem we know that each number is less than 10.
So, x < 10, y < 10 and z < 10.
Therefore, the maximum sum of any two of these numbers, say x + y < 20.
However, statement 1 states x + y + z = 20.
Unless the third number, z in this case, is also positive x + y + z cannot be 20.
Hence, we can conclude that all 3 numbers x, y and z are positive.
Statement 1 ALONE is sufficient.
Eliminate choices B, C and E. Choices narrow down to A or D.
Step 3 of solving this GMAT DS question:
Evaluate Statement (2) ALONE: x + y = 14
As each of x and y is less than 10, both x and y have to be positive for the sum to be 14.
However, z could also be positive or z could be negative.
So, there could be either 2 or 3 positive numbers among the three numbers.
We are not able to find a unique answer from the information in statement 2.
Statement 2 ALONE is NOT sufficient.
Eliminate choice D.
Statement 1 ALONE is sufficient. Choice A is the answer.
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