\textbf{142.} A population with mean $12$ and variance $12/6$, and another population with mean $24$ and variance $7/2$, form a new combined population. If the two populations have equal size, what is the standard deviation of the new population?
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(1) $7/9$ \hfill (2) $3$ \hfill (3) $3/1$ \hfill (4) $3/2$
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