Explanation:

- The average weight of 100 students is 46 kg.

- Let the number of boys be B and the number of girls be G.

- It is given that the average weight of boys is 50 kg and girls is 40 kg.

- Formula: Total weight = Average weight × Number of people.

- Total weight of boys = 50B; Total weight of girls = 40G.

- Total combined weight = 46 × 100.

- Equation: 50B + 40G = 4600.

Now, deriving another equation using B + G = 100:

1. 50B + 40G = 4600

2. Replace G: G = 100 - B

3. 50B + 40(100 - B) = 4600

4. 50B + 4000 - 40B = 4600

5. 10B = 600

6. B = 60 (boys), G = 40 (girls)

- Difference between boys and girls = 60 - 40 = 20.

- Therefore, Option: 3, 20 is correct.