Help with Variance Homework - Find Mean & Probability

[t.war13]

Homework Statement

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.

Homework Equations

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.

 

[Ray Vickson]

Homework Statement

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.

Homework Equations

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.

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.

 

[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?