机器产量方差：25 人总产量标准差

独立和的方差相加：

$$\sigma_T=\sqrt{25}\cdot 60=5\cdot 60=300.$$

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Original Explanation

Let $X_1, \dots, X_{25}$ be the number of pizzas produced by each of the workers per day. Then $T = X_1 + \dots + X_{25}$ gives the total number of pizzas produced per day by the shop. We have that $\text{Var}(T) = 25 \cdot \text{Var}(X_1)$ because of the fact that each of the people are independent and have the same variance. Thus, $\sigma_T = \sqrt{\text{Var}(T)} = 5 \cdot \sigma_{X_1} = 5 \cdot 60 = 300$.