Test 4 Section 6 - #14 (page 597)

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, 3a, 9a, 27a, 81a

...add those up and you get:

a + 3a + 9a + 27a + 81a = 605
121a = 605

...solve for a:

a = 605/121 = 5.