The textbook gives an example of testing a null hypothesis that rolling a die 100 times will give you a value of $6$, $\frac{1}{6}$ times. In the experiment, a die was rolled 100 times and 30 of them were $6$'s.
The book obtains a $z$ score for this with the formula $\frac{\bar{x}- \mu}{ \sqrt{ \frac{p(1-p)}{100}}} = \frac{.30- .167}{ \sqrt{ \frac{.167(1-.167)}{100}}} $.
I understand that $\sqrt{ \frac{0.167(1-0.167)}{100}}$ must be the standard deviation of the sample mean (tell me if I'm wrong), but how did they get $0.167(1-0.167)$ as the variance?
Where did that formula come from?