The nine delegates to the Economic Cooperation Conference include 2 officials from Mexico, 3 officials from Canada, and 4 officials from the United States. During the opening session, three of the delegates fall asleep. Assuming that the three sleepers were determined randomly, the probability that exactly two of the sleepers are from the same country is $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m+n$.
To solve this problem, we need to calculate the probability that exactly two of the three delegates who fall asleep are from the same country. We will break this into cases based on the different combinations of countries the sleepers can be from.

1. **Total number of ways to choose 3 sleepers out of 9 delegates:**
   \[
   \binom{9}{3} = \frac{9!}{3!(9-3)!} = \frac{9 \times 8 \times 7}{3 \times 2 \times 1} = 84
   \]

2. **Calculate the number of favorable outcomes for each case:**

   - **Case 1: Two from Mexico (M) and one from Canada (C):**
     \[
     \binom{2}{2} \times \binom{3}{1} = 1 \times 3 = 3
     \]

   - **Case 2: Two from Mexico (M) and one from the United States (U):**
     \[
     \binom{2}{2} \times \binom{4}{1} = 1 \times 4 = 4
     \]

   - **Case 3: Two from Canada (C) and one from Mexico (M):**
     \[
     \binom{3}{2} \times \binom{2}{1} = 3 \times 2 = 6
     \]

   - **Case 4: Two from Canada (C) and one from the United States (U):**
     \[
     \binom{3}{2} \times \binom{4}{1} = 3 \times 4 = 12
     \]

   - **Case 5: Two from the United States (U) and one from Mexico (M):**
     \[
     \binom{4}{2} \times \binom{2}{1} = 6 \times 2 = 12
     \]

   - **Case 6: Two from the United States (U) and one from Canada (C):**
     \[
     \binom{4}{2} \times \binom{3}{1} = 6 \times 3 = 18
     \]

3. **Sum the number of favorable outcomes:**
   \[
   3 + 4 + 6 + 12 + 12 + 18 = 55
   \]

4. **Calculate the probability:**
   \[
   \text{Probability} = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}} = \frac{55}{84}
   \]

5. **Simplify the fraction:**
   The fraction \(\frac{55}{84}\) is already in its simplest form because 55 and 84 have no common factors other than 1.

6. **Identify \(m\) and \(n\):**
   Here, \(m = 55\) and \(n = 84\).

7. **Calculate \(m + n\):**
   \[
   m + n = 55 + 84 = 139
   \]

The final answer is \(\boxed{139}\)