Great question — this is a classic seat-arrangement problem with a constraint, and it's one where students consistently lose points by undercounting or overcounting. Let me break it down carefully.
Key concept being tested: Constrained Permutations
The setup: 7 people (Noah, Mia, and 5 friends — call them A, B, C, D, E) need to be seated in 7 adjacent seats, with the constraint that exactly one friend sits between Noah and Mia.
Step 1: Find the number of valid position-sets for the Noah–[friend]–Mia block.
We need three consecutive seats where Noah is in seat 1, one friend is in seat 2, and Mia is in seat 3 (or vice versa). The three-seat window can start at position 1, 2, 3, 4, or 5 in a row of 7:
(1,2,3), (2,3,4), (3,4,5), (4,5,6), (5,6,7) → 5 position windows
Step 2: Arrange Noah and Mia within the block.
Noah can be on the left with Mia on the right, or Mia on the left with Noah on the right. That's 2 ways.
Step 3: Choose which friend sits in the middle.
Any 1 of the 5 remaining friends can be the person sitting between them → 5C1 = 5 ways
Step 4: Arrange the remaining 4 friends in the other 4 seats.
The remaining 4 people can sit in any order → 4! = 24 ways
Step 5: Multiply everything together.
5 × 2 × 5 × 24 = 1,200
Answer: D
Common trap: Many students treat the Noah–friend–Mia group as a single block and then arrange 5 units in a row (giving 5! = 120 as the block positions), forgetting that the block itself is 3 seats wide within a fixed 7-seat row — not a circular or flexible arrangement. The block can only start in 5 specific positions, not slide freely.
Takeaway: When you have a "sandwich" constraint (X is between A and B), count the valid windows for the trio explicitly, then multiply by the internal arrangements of the trio and the external arrangements of everyone else.