Average of first $10$ multiples of $3$ is:

Average of first $10$ multiples of $3$ is:
  1. $15$
  2. $16.5$
  3. $18$
  4. $20$

Solution

First $10$ multiples of $3$: $3,6,9,\ldots,30$. Sum $= 3(1+2+\cdots+10) = 3 \cdot 55 = 165$. Average $= \dfrac{165}{10} = 16.5$.

Asked in: IMO

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