Given:

• Average weight of 9 persons = $50$ kg
• Average weight of first 5 persons = $45$ kg
• Average weight of last 5 persons = $55$ kg

Step 1: Total weight of 9 persons = $9 \times 50 = 450$ kg Step 2: Total weight of first 5 persons = $5 \times 45 = 225$ kg Step 3: Total weight of last 5 persons = $5 \times 55 = 275$ kg

Let the weight of the 5th person be $x$.

Step 4: Total weight of all 9 persons = $(225 + 275 - x) = 450$ Step 5: Simplifying: $500 - x = 450$ Step 6: $x = 50$

Conclusion: The weight of the 5th person is $50$ kg.