Don't immediately dive into algebra, substituting one equation into the next in a complicated cascade to try to get down to one variable. You can solve this way—but you'll need so many steps that you're a lot more likely to make mistakes (not to mention, waste a bunch of time).

Take a deep breath and examine this setup before you start trying to solve. The problem asks for *x* + *y* + *z*. There are three equations. It's a pain to try to combine just two of the equations (for example, if you subtract the second equation from the first one, you'll get the *x* to drop out...but you'll still be left with the two variables *y* and *z*).

Treat this as a Combo (short for combination of variables) problem. Is there a way to solve directly for *x* + *y* + *z*, all at once?

You need to end up with the same number of *x*'s and *y*'s and *z*'s in the mix and they need to be added together. Across all three equations combined, there are a total of two *x*'s, two *y*'s, and two *z*'s—that is, the same number of *x*'s and *y*'s and *z*'s, exactly what you need. Try adding up all three equations at once. What happens?

2*x* + 2*y* + 2*z* = 26

*x* + *y* + *z* = 13

Done! The correct answer is (C).

The test writers are looking for ways to test your ability to think critically and logically about quant topics. Start building a habit of actually thinking about what the problem has given you and what it's asking you to find—before you start frantically doing a bunch of textbook algebra.