**Note: In early editions of the Blue Book, the answer key is WRONG here. Even though the answer key says it's 6, I assure you the answer is 5.**

Even though they're describing a series here, treat it like a function:

*just follow the rules*. According to the rules laid out in the question, the the first 5 terms are going to be:

*a*, 3

*a*, 9

*a*, 27

*a*, 81

*a*

...add those up and you get:

*a*+ 3

*a*+ 9

*a*+ 27

*a*+ 81

*a*= 605

121

*a*= 605

...solve for

*a*:

*a*= 605/121 = 5.

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