Verified answer

Answer:

D. 10

Step-by-step explanation:

a1 = 9

an = 27

Sn = 180

Sn = n/2 (a1 + an)

180 = n/2 (9 + 27)

180 = n/2 (36)

180 = 36n/2

2(180) = (36n/2)2

*Multiply both sides by 2 to get rid of the denominator of the right side

360 = 36n

*Divide both sides by 36 to get rid of the numerical coefficient of n

10 = n

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