In a group of 10 batteries, 5 are dead. you choose 2 batteries at random. a) create a probability model for the number of good batteries you get. b) what's the expected number of good ones you get? c) what's the standard deviation?

You have 10 batteries of which 5 are dead. That means 1/2 of the batteries are dead.  So if you randomly select a battery, there is a 50% chance of selecting a dead one, or a good one.  But since you're choosing 2, that means the first one you select has a 50% chance of being dead or good, (5/10), but then the second one has a 44% chance of being dead or good, (4/9).  multiplying these, you get 22% chance of randomly selecting 2 good batteries.  

true

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6/20

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