Explanation:

- We have seven persons: M, N, O, P, Q, R, and S on seven floors.

- N lives two floors above R. So, N = R + 2.

- M doesn't live on a prime numbered floor (floors 2, 3, 5, and 7 are prime).

- Three persons live between R and P's floors, so if R is on floor 1, P is on floor 5 or vice versa.

- O lives just above S.

- P lives on a prime numbered floor.

- Q lives above M.

Given all this, we can determine:

- Place R on floor 2. Then, N will be on floor 4.

- P should be on a prime numbered floor and three floors away from R, so P is on floor 5.

- O is above S, we can place O on floor 7 and S on floor 6.

- Since Q is above M, we place Q on floor 3, leaving floor 1 for M.

The number of persons below R (floor 2) is 1, thus there should be 1 person above N (floor 4).

- Correct Answer: 3, N.

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