But i really need a name of this book and authors :)
It might be c. im not really sure, but that's what I got. sorry if its wrong...
Answer:
-55. just multiple -5 by 11
After having done this on paper:

We want to isolate the variable, and the first step is to subtract the 8.

Since the <em>n</em> is being multiplied by 3, and that = 8, we can multiply both sides by 3.
<em>n</em> = 24
Let me know if this helped you understand better.
Also, you can check this by substituting in 24 for <em>n</em>.
This is a trick question. They eat 1/3 of the gallons given. Multiply
1/3 * 1/3 = 1/9 gallons or 0.111 gallons