# SAT Math Multiple Choice Practice Question 512: Answer and Explanation

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**Question: 512**

**11.**

If *PQRS* is a parallelogram, then which of the following must be equal to *x*?

A. 180 – *b*

B. 180 – *c*

C. *a* + *b*

D. *a* + *c*

E. *b* + *c*

**Correct Answer:** E

**Explanation:**

**E** There are variables in the answer choices, so plug in. However, you can't plug in a value for all the variables at once because you must follow the rules of geometry. (Makes sense, right? It's the last question in the section.) Let's start by saying *a* = 70 and *b* = 60. Because *PQRS* is a parallelogram, angle *Q* must equal angle *S*, so angle *S* also equals 60. If you look at the big triangle that contains *a* and *c*, you already know that two of the angles are 60 and 70, so the third angle, *c*, must be 50. You know that *PQ* and *SR* are parallel and, you can see that *x* is a big angle and *a* is a small angle. So *a* + *x* = 180. Because *a* = 70, that means *x* = 110. Therefore, your target answer is 110. Plug your values for *a*, *b*, and *c* into the answers and you'll find that E equals 110.