GMAT Math : DSQ: Solving linear equations with two unknowns
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Example Questions
Example Question #919 : Data Sufficiency Questions
A toy store sells dolls for 
(1) Last week, the store sold twice as many trucks as dolls.
(2) Last week, the store made 
A: Statement (1) ALONE is sufficient, but statement (2) ALONE is not sufficient
D: EACH statement ALONE is sufficient
E: Statements (1) and (2) TOGETHER are not sufficient
C: BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
B: Statement (2) ALONE is sufficient, but statement (1) ALONE is not sufficient
C: BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
Let:
t: the number of trucks sold last week
d: the number of dolls sold last week - d is the value we are looking to find
To evaluate the statements, we translate the word problems into equations
(1): 
(2):
Each statement provides a single equation with two unknowns which is unsolvable, so each statement alone is not enough.
The two statements taken together give us a system of two equations with two unknowns, which we can solve.
Therefore the right answer is C.
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