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60 vote

What is the average of first 30 multiples of 9?

Asked on by | Votes 60 | Views: 4311 | Tags: average     | quantitative aptitude     | Add Bounty

What is the average of first 30 multiples of 9?
1). 142
2). 138.5
3). 139.5
4). 143.5


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2 answers

38 vote
Answered by on | Votes 38 |

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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18 vote
Answered by on | Votes 18 |
no. of terms = ( n-1 )/2 +1 then ,{ ( 30 -1) /2 }+1 = 14.5 +1 = 15.5 then we simply multiply with 9 , then we will find answer . So , 15.5 × 9 = 139.5




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