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# What is the value of xy - yz? (1) y = 2 (2) x - z = 5

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Re: What is the value of xy - yz? (1) y = 2 (2) x - z = 5 [#permalink]
The question has three variables.
A has the value of only y..since we don't have the values of x and z, we cannot find the value of y(x-z). So Insufficient
B gives the value of x-z but not y. So Insufficient.
But together, we can find y(x-z) using both A and B. So C

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Re: What is the value of xy - yz? (1) y = 2 (2) x - z = 5 [#permalink]
Bunuel wrote:
What is the value of xy - yz?

(1) y = 2

(2) x - z = 5

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.
The question $$xy - yz$$ is equivalent to $$y(x-z)$$ by factoring.

Since we have 3 variables ($$x, y$$ and $$z$$) and 0 equations, E is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)

$$xy - yz = y(x-z) = 2 \cdot 5 = 10$$ since we have $$y = 2$$ and $$x - z = 5$$.

Since both conditions together yield a unique solution, they are sufficient.