Expected Value and Standard Deviation of A1 Computer's Rebate

[geforce]

A1 Computer has 15 tablets stocked, but four of them were actually defectives.
A client bought two tablets from A1 Computer. If both of them are good, things are fine.
If the client gets one defective machine, A1 Computer will replace it and give a $100 rebate
to the client. If the clients gets two defective machines, A1 Computer will replace both of
them and give a $1000 rebate to the client. What is the expected value and the standard
deviation of the company's rebate?

P(R) = 4/15 x 100 = 26.6%

Since there were 4 defective machines then 2 x 1000 = 2000 was given away to client(s)
so value = 2000
how would I find the standard deviation? I know the usual formula but where would i plug into it?

 

[HallsofIvy]

I think you are completely misunderstanding the problem.

You calculated P(R), the probability that a given computer is bad. Now What is the probability that, of 2 computers chosen at random from those 15
1) neither is bad.
2) one is bad.
3) both are bad.

 

[Ray Vickson]

A1 Computer has 15 tablets stocked, but four of them were actually defectives.
A client bought two tablets from A1 Computer. If both of them are good, things are fine.
If the client gets one defective machine, A1 Computer will replace it and give a $100 rebate
to the client. If the clients gets two defective machines, A1 Computer will replace both of
them and give a $1000 rebate to the client. What is the expected value and the standard
deviation of the company's rebate?

P(R) = 4/15 x 100 = 26.6%

Since there were 4 defective machines then 2 x 1000 = 2000 was given away to client(s)
so value = 2000
how would I find the standard deviation? I know the usual formula but where would i plug into it?

I don't understand. If you know the formula, what is preventing you from substituting in the appropriate numbers and doing the arithmetic? Or, are you saying that you don't know what numbers to plug in?

RGV