A certain number of people are sitting in a row, facing north. N is sitting third to the left of P. M is sitting third to the left of N. O is sitting third to the left of M. M is sitting between O and N. N is sitting between M and P. If no other person is sitting in the row, what is the total number of persons seated?

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**Solution:**

1. Let us arrange the persons in a row facing north.

2. Given: N is sitting third to the left of P.
   - This means there are two persons between N and P, with N to the left of P.

3. M is sitting third to the left of N.
   - Similarly, two persons between M and N, with M to the left of N.

4. O is sitting third to the left of M.
   - Two persons between O and M, with O to the left of M.

5. M is sitting between O and N.
   - This confirms the order: O, M, N.

6. N is sitting between M and P.
   - This confirms the order: M, N, P.

7. Combining all, the order from left to right is:
   O, (person), (person), M, (person), N, (person), P

8. Counting the positions:
   - O at position 1
   - Two persons between O and M: positions 2 and 3
   - M at position 4
   - One person between M and N: position 5
   - N at position 6
   - One person between N and P: position 7
   - P at position 8

9. However, the problem states M is third to the left of N, so there must be exactly two persons between M and N, not one.

10. Re-examining the arrangement with correct spacing:
    - N is third to the left of P: N is 3rd left of P means two persons between N and P.
    - M is third to the left of N: two persons between M and N.
    - O is third to the left of M: two persons between O and M.

11. So the sequence with exact spacing is:
    O, _, _, M, _, _, N, _, _, P

12. Counting total persons:
    - O at position 1
    - Two persons between O and M: positions 2,3
    - M at position 4
    - Two persons between M and N: positions 5,6
    - N at position 7
    - Two persons between N and P: positions 8,9
    - P at position 10

13. Total persons = 10

**Answer:** 10

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