Quiz or test

I have a quiz with 20 questions. Each question has four multiple-choice answers, A, B, C, D. THERE IS NO WAY TO KNOW THE CORRECT ANSWER OF ANY GIVEN QUESTION, but the answers are static, in that if the "correct" answer to #1 = C, then it will always be equal to C. You can retake the quiz as many times as you want. You take the quiz the first time, making a log of all of your selections as you go (e.g., 1=C, 2=A, 3=D, 4=A, 5=A, 6=C, . . . . 19=A, 20=C). .. When complete, you learn you got 8 of 20 correct. So you take it again, and again log all of your answers, some different, some the same, and when done, you learn you got nine correct. You can retake it as often as you like. Using only guesses, what is the fastest way you could complete the test and get 100% correct, and what method would you use?

Correct answer:

n =  4649045868

Step-by-step explanation:

$n=4\cdot 3^{20-1}=4649045868=4.649\cdot 10^9$

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