AMC 8 1995 Problem 23

AMC8 1995 年第 23 题

基本计数原理★★★☆☆

How many four-digit whole numbers are there such that the leftmost digit is odd, the second digit is even, and all four digits are different?

有多少个四位数满足：最左边一位是奇数，第二位是偶数，且四位数字各不相同？

A.1120
B.1400
C.1800
D.2025
E.2500

正确答案 · Correct Answer

解析 · Solution

Solution 1

Count from left to right: There are 5 choices for the first digit, 5 choices for the second, 8 remaining choices for the third, and 7 remaining for the fourth, so there are

5*5*8*7= \text{(B)1400}

numbers.

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