Standard deviation of a new sample

seidjeep
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Hello all,

I am trying to figure out the following question with no luck: I have a box of resistors, mean 100 ohms and SD of 20 ohms. I form a subgroup of these resistors with mean of 100 ohms and SD of 5 ohms. Find the standard deviation of the remaining sample. The question does not give starting number of resistors. I'm guessing the standard deviation would be greater than the original 20 ohms since we removed all the resistors closest to the mean. Any clues about how to get this started would be awesome. Thanks.
 
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The key to this is knowing how many in the original set and how many have been set aside into the subset. It would be a straightforward calculation as a function of these parameters.
 

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