Find the mean and variance for the first $10$ multiples of $3$.

The first $10$ multiples of $3$ are $3, 6, 9, 12, 15, 18, 21, 24, 27, 30$.
Here,the number of observations is $n = 10$.
The mean $\bar{x}$ is calculated as:
$\bar{x} = \frac{\sum_{i=1}^{10} x_i}{10} = \frac{3+6+9+12+15+18+21+24+27+30}{10} = \frac{165}{10} = 16.5$.
The following table shows the calculation for variance:

$x_i$	$(x_i - \bar{x})^2$
$3$	$182.25$
$6$	$110.25$
$9$	$56.25$
$12$	$20.25$
$15$	$2.25$
$18$	$2.25$
$21$	$20.25$
$24$	$56.25$
$27$	$110.25$
$30$	$182.25$

The sum of squares $\sum (x_i - \bar{x})^2 = 742.5$.
Variance $(\sigma^2) = \frac{1}{n} \sum (x_i - \bar{x})^2 = \frac{742.5}{10} = 74.25$.