Suppose the following data are obtained by recording $X$, the number of customers that arrive at an automatic banking machine during $15$ successive one-minute time intervals.
2 1 3 2 0 1 4 2
0 2 3 1 0 0 4
The formula for sample variance is given by
$$s^2 = \frac{1}{n-1}\sum_{i=1}^{n} (x_i - \bar{x})$$
where $\bar{x} = 25/15$
I got $$\frac{1}{14}\left[(2-25/15)^2 + (1-25/15)^2 + (3-25/15)^2 \\ {}+ (2-25/15)^2 + (1-25/15)^2 + (4-25/15)^2 \\ {}+ (2-25/15)^2 + (2-25/15)^2 + (3-25/15)^2 \\ {}+ (1-25/15)^2 + (4-25/15)^2\right] \approx 1.15$$
whereas the answer is $1.94$ . What am I doing wrong here?
You left out the points with value $0$, they do contribute to the variance because the mean is not zero.
When you do it right, you get $$\frac1{14}\frac{246}{9} \approx 1.952$$