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Moderate

Finding the Standard Deviation of $X-Y$

APSTAT-FBXXZB

A random variable $X$ has a mean of $40$ and standard deviation of $7$. A random variable $Y$ has a mean of $30$ and a standard deviation of $5$.

If $X$ and $Y$ are independent, approximately what is the standard deviation of $X-Y$?

A

$74$

B

$12$

C

$8.6$

D

$6.0$

E

$2$