Quantitative Aptitude Ques 555

Question: The average of the first nine integral multiples of 3 is

Options:

A) 12

B) 15

C) 18

D) 21

Show Answer

Answer:

Correct Answer: B

Solution:

  • By using arithmetic series Sum of first 9 integral multiples of 3 $ S _{n}=\frac{n}{2}[2a+(n-1),d] $ where, $ n=9, $ $ a=3 $ and $ d=3 $

$ \therefore $ $ S _{n}=\frac{9}{2}[2\times 3+(9-1),3]=\frac{9}{2}\times 30=135 $

$ \therefore $ Average $ =\frac{135}{9}=15 $ Alternate Method Average of n multiples of any number $ =\frac{Number\times (n+1)}{2}=\frac{3\times (9+1)}{2}=\frac{3\times 10}{2}=15 $

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