0.39 What is the average of first 30 multiples of 9?
A. 142
B. 138.5
C. 139.5
D. 143.5
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Sum of the nth term = n/2 × (2a + (n – 1) × d)
n = number of terms
a = first term
d = Common difference
Sum of the 30th terms = 30/2 × (2 × 9 + (30 – 1) × 9) (n = 30, a = 9, d = 9)
= 15 × (18 + 29 × 9)
= 4185
Sum of the first 30 multiples of 9 = 4185
∴ Average of the first 30 multiples of 9 = 4185/30 = 139.5
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