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PUMaC 2008 · 组合（B 组） · 第 2 题

PUMaC 2008 — Combinatorics (Division B) — Problem 2

专题
Discrete Math / 离散数学
难度
L3
来源
PUMaC

题目详情

  1. (2 points) How many 3-digit numbers contain the digit 7 exactly once?
解析
  1. How many 3-digit numbers contain the digit 7 exactly once? ( ANS: 225: if the 7 is the first digit, there are 81 choices for the other two digits, if the 7 is second or third, there are 72 choices, as a three digit number cannot start with 0. None of these numbers are the same, as the 7 is in a different place each time. CB: ACH, RH, PR)