Question

$20$ 个数的平均数为 $30$ ，另外 $30$ 个数的平均数为 $20$ 。这 $50$ 个数的平均数是多少？

The average (mean) of $20$ numbers is $30,$ and the average of $30$ other numbers is $20.$ What is the average of all $50$ numbers?

$23$

$24$

$25$

$26$

$27$

两组数的总和为 $20 \cdot 30 + 30 \cdot 20 = 600 + 600$ $= 1200$ 。所以这 $50$ 个数的平均数为 $\dfrac{1200}{50} = 24$ 。

所以正确答案是 B。

The combined sum is $20 \cdot 30 + 30 \cdot 20 = 600 + 600$ $= 1200.$ The average of all $50$ numbers is $\dfrac{1200}{50} = 24.$

Thus, the correct answer is B.