Explanation:

- In a class of 80 students, 60% are girls, so 48 are girls (0.6 × 80) and 32 are boys.

- The average weight of boys is 5% more than that of girls. Let's say the average weight of girls is G kg. Then the average weight of boys would be 1.05G kg.

- The overall average weight is 51 kg. Using the average formula: $$(48G+32\times 1.05G)/80=51$$

- Solving this gives: $$(48G+33.6G)=80\times 51$$

- This simplifies to: $$81.6G=4080$$

- Solving for G, we find: $$G=50$$

- Option 2: 50 kg is the correct answer.

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