# Statistics

A jar contains 7 white marbles and 3 blue marbles. Given that the 4 marbles are chosen from the jar at the same time, find the standard deviation of the number of the blue marbles chosen. If the standard deviation can be written as $$\dfrac{ \sqrt{a} } { b}$$ where $$a$$ and $$b$$ are coprime positive integers and $$a$$ is square-free, then what is $$a+b$$?

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