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خطا

$\begin{equation} \left.\begin{aligned}   \overline X  + 2\sigma  = 230 \\    \overline X  - 2\sigma  = 220       \end{aligned} \right\} \end{equation}  \Rightarrow 4\sigma  = 10 \Rightarrow \sigma  = \frac{{10}}{4} = \frac{5}{2} \Rightarrow {\sigma ^2} = \frac{{25}}{4} = 6/25$