Maths Olympiad Prep

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Combinatorics Difficulty 4.4 AIME Find the answer United States

Problem:

Five people of heights 65,66,67,6865, 66, 67, 68, and 6969 inches stand facing forwards in a line. How many orders are there for them to line up, if no person can stand immediately before or after someone who is exactly 11 inch taller or exactly 11 inch shorter than himself?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

Answer: 1414

Let the people be A,B,C,D,EA, B, C, D, E so that their heights are in that order, with AA the tallest and EE the shortest. We will do casework based on the position of CC.

- Case 1: CC is in the middle. Then, BB must be on one of the two ends, for two choices. This leaves only one choice for DD—the other end. Then, we know the positions of AA and EE since AA cannot neighbor BB and EE cannot neighbor DD. So we have 22 options for this case.

- Case 2: CC is in the second or fourth spot. Then, we have two choices for the position of CC. Without loss of generality, let CC be in the second spot. Then, the first and third spots must be AA and EE, giving us two options. This fixes the positions of BB and DD, so we have a total of 2×2=42 \times 2 = 4 options for this case.

- Case 3: CC is in the first or last spot. Then, we have two choices for the position of CC. Without loss of generality, let it be in the first spot. Either AA or EE is in the second spot, giving us two choices. Without loss of generality, let it be AA. Then, if DD is in the third spot, the positions of BB and EE are fixed. If EE is in third spot, the positions of BB and DD are fixed, so we have a total of 2×2×(1+1)=82 \times 2 \times (1+1) = 8 options for this case.

Hence, we have a total of 2+4+8=142 + 4 + 8 = 14 possibilities.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.