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The mathematical knowledge and skill required solving Data Sufficiency
problems is no greater than that required to solve standard math problems.
What make Data Sufficiency problems appear more difficult at first is the
complicated directions. But once you become familiar with the directions,
you'll find these problems no harder than standard math problems. In fact,
students usually become proficient more quickly on Data Sufficiency
problems over time. Here are some tips:

Unwarranted Assumptions

• Be extra careful not to read any more into a statement than what is
given.

• The main purpose of some difficult problems is to lure you into
making an unwarranted assumption.

Elimination
Data Sufficiency questions provide
fertile ground for elimination. In fact, it is rare that you won't be able
to eliminate some answer-choices. Remember, if you can eliminate at least
one answer choice, the odds of gaining points by guessing are in your
favor.

Analysis
Ask yourself what specific information you
need in order to answer the question posed. As you consider each of the
two numbered statements independent of each other, ask yourself whether
the statement provides any such information.

This problem involves the concept of proportion. Notice that no
arithmetical calculations are required here. That's because Data
Sufficiency problems are designed to test you primarily on concepts, not
on your ability to solve a problem by working to a quantitative solution.
(That's what Problem Solving questions are for.)

A good way to see this to use a decision tree.

DIRECTIONSfor Data Sufficiency
questions: Each of the data sufficiency problems below consists
of a question and two statements, labeled I and II, 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 July or the meaning of counterclockwise), you are to
blacken space.

(A) if statement I BY ITSELF is sufficient;
(B) if statement II BY
ITSELF is sufficient;
(C) if statements I and II TAKEN TOGETHER are
sufficient;
(D) if EITHER statement by ITSELF is sufficient;
(E) if
I and II TOGETHER are NOT SUFFICIENT.

Before starting with Data Sufficiency questions revise the
directions.

ELIMINATION: GMAT Data Sufficiency questions provide
fertile ground for elimination. In fact, it is rare that you won't be able
to eliminate some answer-choices. Remember, if you can eliminate at least
one answer choice, the odds of gaining points by guessing are in your
favor. The following table summarizes how elimination functions with Data
Sufficiency problems.

1. What are the two numbers x and y?

I. Sum of x and y is 528.
II. HCF of x and y is 33.

Solution: Each statement alone is not sufficient.

Combining I and II: As HCF of x and y is 33.
Let the two numbers be
33a and 33b.
33a + 33b = 528 Therefore, a + b = 16.
Now, two
co-primes with sum 16 are (1, 15), (3, 13), (5, 11) and (7, 9)
Thus,
the numbers could be (33, 415) or (99, 429) and so on.
Thus, the value
of x and y cannot be uniquely found out. Hence, the question cannot be
answered. Hence, (E).

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