Calculating mean and variance of probability

[bunnyrabid]

Homework Statement

Given a 30% failure rate, what is the mean number of students who fail, and what is the variance? There are 8 students in the course.

Homework Equations

I think these are the correct equations:
Mean = Probability * Number of students
Variance = Sum from i=1 to 8 of p_i(x_i-Mean)^2

The Attempt at a Solution

So using the Mean formula I got 0.3*8=2.4
Using the variance formula I got
(0.3)(1-2.4)²+(0.3)(2-2.4)²+(0.3)(3-2.4)²+(0.3)(4-2.4)²+(0.3)(5-2.4)²+(0.3)(6-2.4)²+(0.3)(7-2.4)²+(0.3)(8-2.4)²
which is approximately equal to 23.184 (see variance formula here: wolfram alpha, then click approximate form )
Do I then square root to find the actual variance?

Thanks for any help!

 

[lanedance]

the pi in your variance equation are not correct, you need to have a think about the probability distribution