Question

下方每个框中的数是它上方一行中与它相接的两个框中数字的乘积。例如， $30 = 6\times5$ 。

顶行缺失的数是多少？

The number in each box below is the product of the numbers in the two boxes that touch it in the row above. For example, $30 = 6\times5.$

What is the missing number in the top row?

$2$

$3$

$4$

$5$

$6$

知识点：逆推法

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第二行两个数的乘积是 $600$ ，所以第二行右侧缺失的数是 $600 \div 30 = 20$ 。

现在知道 $5$ 与第一行缺失数的乘积为 $20$ 。因此缺失数是 $20 \div 5 = 4$ 。

所以正确答案是 C。

The product of the numbers in the second row is $600,$ so the missing number in the middle right box is $600 \div 30 = 20.$

Now we know that the product of $5$ and the missing number in the first row is $20.$ Therefore, the missing number is $20 \div 5 = 4.$

Thus, C is the correct answer.