Question

有许多两位数是 $7$ 的倍数，但其中只有两个倍数的数位和为 $10$ 。这两个 $7$ 的倍数之和是

There are many two-digit multiples of $7,$ but only two of the multiples have a digit sum of $10.$ The sum of these two multiples of $7$ is

$119$

$126$

$140$

$175$

$189$

答案：A

知识点：倍数数字系统列举

难度评级：730

解答：

$7$ 的两位数倍数为其中数位和为 $10$ 的是 $28$ 和 $91$ 。 $14, 21, 28, 35, 42, 49, 56, 63, 70,$ $77, 84, 91, 98.$

它们的和是 $28 + 91 = 119$ 。

所以正确答案是 A。

Listing out all the two-digit multiples of $7,$ we get $14, 21, 28, 35, 42, 49, 56, 63, 70,$ $77, 84, 91, 98.$ We have that $28$ and $91$ are the two multiples whose digits add to $10.$

The sum of these numbers is $28 + 91 = 119.$

Thus, A is the correct answer.

1997 AMC 8 第 5 题

答案：A

难度评级：730

解答：

其他年份的第 5 题