Help with Variance

1. Jan 24, 2013

t.war13

1. The problem statement, all variables and given/known data

For every bike that comes into a repair shop there is a 0.1 probability that a wheel will need replacing. The shop always has 3 wheels in stock. X is the number of bikes that can be repaired before the wheels run out. I need to figure out the mean and variance of X.

2. Relevant equations

3. The attempt at a solution

For the mean I have done : 1/(0.1^3) = 1000 ..is this correct?
How would I figure out the variance?

I also need to figure out the probability that 20 or more bikes can be fixed before the wheels run out. I have done (1-0.001*19) = 0.981. I don't think that this is correct though?

Thanks for any help.

2. Jan 24, 2013

Ray Vickson

Your expression for the mean is wrong. How did you obtain it? We cannot help if you don't show your work, because we will have no way to tell where you went astray.

3. Jan 24, 2013

haruspex

(Ray, my guess is there was no working.)
t.war13, what is the probability that no more than two of the next N bikes will need wheels?