Question

Find the mean of first five multiples of $3.$

Answer 1

The first five multiples of $3$ are $3,6,9,12$ and $15$.
Let $\overline{x}$ denote their arithmetic mean.
Then $\overline{x}=\frac{\text{sum of the terms}}{\text{number of terms}}$
$=\frac{3+6+9+12+15}{5}$
$=\frac{45}{5}$
$\therefore \overline{x}=9$