# SAT Math Multiple Choice Question 144: Answer and Explanation

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**Question: 144**

**9.** If 8x + 8y = 18 and x^{2} - y^{2} = - , what is the value of 2x - 2y ?

- A. -
- B. -
- C.
- D.

**Correct Answer:** A

**Explanation:**

A Start by simplifying 8x + 8y = 18 by dividing each term by 8: x + y = or : x + y = . The second equation provided in the question can be factored: x^{2} - y^{2} is the same as (x + y)(x - y), so the second equation can also be written (x + y)(x - y) = -. Since you know that x + y = , you can rewrite the second equation as (x - y) = -. Multiply both sides by : : x - y = -() or x - y = -. Since the question asks for the value of 2x - 2y, simply multiply everything by 2: 2(x - y) = 2(-) = -, which is (A).