Chúng ta sẽ đổi từng đơn vị theo các tỷ lệ sau:

- 1 cm² = 100 mm²
- 1 dm² = 10000 mm²
- 1 m² = 1000000 mm²

Dựa vào các tỷ lệ này, ta tính toán như sau:

1. \( 2 \, \text{cm}^2 = 2 \times 100 = 200 \, \text{mm}^2 \)

2. \( 7 \, \text{dm}^2 = 7 \times 10000 = 70000 \, \text{mm}^2 \)

3. \( 6200 \, \text{mm}^2 = \frac{6200}{100} = 62 \, \text{cm}^2 \)

4. \( 90000 \, \text{mm}^2 = \frac{90000}{10000} = 9 \, \text{dm}^2 \)

5. \( 6 \, \text{dm}^2 + 15 \, \text{cm}^2 = 6 \times 10000 + 15 \times 100 = 60000 + 1500 = 61500 \, \text{mm}^2 \)

6. \( 3050 \, \text{mm}^2 = \frac{3050}{100} = 30.5 \, \text{cm}^2 = 3050 \, \text{mm}^2 \) (dẫn chiếu lại)

7. \( 15 \, \text{dm}^2 + 7 \, \text{mm}^2 = 15 \times 10000 + 7 = 150000 + 7 = 150007 \, \text{mm}^2 \)

8. \( 2 \, \text{m}^2 + 8 \, \text{dm}^2 = 2 \times 1000000 + 8 \times 10000 = 2000000 + 80000 = 2080000 \, \text{mm}^2 \)

9. \( 4 \, \text{cm}^2 + 685 \, \text{mm}^2 = 4 \times 100 + 685 = 400 + 685 = 1085 \, \text{mm}^2 \)

Tóm lại:

1. \( 2 \, \text{cm}^2 = 200 \, \text{mm}^2 \)
2. \( 7 \, \text{dm}^2 = 70000 \, \text{mm}^2 \)
3. \( 6200 \, \text{mm}^2 = 62 \, \text{cm}^2 \)
4. \( 90000 \, \text{mm}^2 = 9 \, \text{dm}^2 \)
5. \( 6 \, \text{dm}^2 + 15 \, \text{cm}^2 = 61500 \, \text{mm}^2 \)
6. \( 3050 \, \text{mm}^2 = 30.5 \, \text{cm}^2 = 3050 \, \text{mm}^2 \)
7. \( 15 \, \text{dm}^2 + 7 \, \text{mm}^2 = 150007 \, \text{mm}^2 \)
8. \( 2 \, \text{m}^2 + 8 \, \text{dm}^2 = 2080000 \, \text{mm}^2 \)
9. \( 4 \, \text{cm}^2 + 685 \, \text{mm}^2 = 1085 \, \text{mm}^2 \)