Explanation:

- Initially, the total weight of 30 students is 30 * 42 = 1260 kg.

- When Sita joins, the average becomes 41 kg for 31 students.

- Total weight with Sita: 31 * 41 = 1271 kg.

- Sita's weight: 1271 - 1260 = 11 kg.

- When Gita joins, the average becomes 43 kg for 31 students.

- Total weight with Gita: 31 * 43 = 1333 kg.

- Gita's weight: 1333 - 1260 = 73 kg.

- Average weight of Sita and Gita: (11 + 73) / 2 = 42 kg.

Answer: 42 kg