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## 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.