Question

有多少个两位正整数至少有一个数字是 $7$ ？

How many two-digit positive integers have at least one $7$ as a digit?

$10$

$18$

$19$

$20$

$30$

$70$ 到 $79$ 给出 $10$ 个十位为 $7$ 的两位数。

数列 $17, 27, \ldots, 97$ 给出 $9$ 个个位为 $7$ 的两位数。

因为 $77$ 被重复计算一次，所以总数为 $10 + 9 - 1 = 18$ 。

所以正确答案是 B。

The numbers $70$ through $79$ give $10$ with a $7$ in the tens place.

The numbers $17, 27, \ldots, 97$ give $9$ with a $7$ in the units place.

Since $77$ is counted twice, the total is $10 + 9 - 1 = 18.$

Thus, the correct answer is B.