Maths Olympiad Prep

Track / Stage 3 / 118 of 260 #118 of 1964

Problem 118

AMC 10/12, early questions
Combinatorics Difficulty 3.3 Multiple choice

How many 4-digit positive integers have four different digits, where the leading digit is not zero, the integer is a multiple of 5, and 5 is the largest digit?

Pick one

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Official solution

We can separate this into two cases. If an integer is a multiple of 5,5, the last digit must be either 00 or 5.5.
Case 1: The last digit is 5.5. The leading digit can be 1,2,3,1,2,3, or 4.4. Because the second digit can be 00 but not the leading digit, there are also 44 choices. The third digit cannot be the leading digit or the second digit, so there are 33 choices. The number of integers is this case is 4431=48.4\cdot4\cdot3\cdot1=48.
Case 2: The last digit is 0.0. Because 55 is the largest digit, one of the remaining three digits must be 5.5. There are 33 ways to choose which digit should be 5.5. The remaining digits can be 1,2,3,1,2,3, or 4,4, but since they have to be different there are 434\cdot3 ways to choose. The number of integers in this case is 1343=36.1\cdot3\cdot4\cdot3=36.
Therefore, the answer is 48+36=(D) 8448+36=\boxed{\textbf{(D)}\ 84}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.