Explanation:

- The average weight of the entire class is 55 kg.

- The ratio of the average weight of boys to girls is given as 5:4.

- 40% of the students are boys, and therefore, 60% are girls.

- Let's denote the average weight of boys as 5x and that of girls as 4x (based on their given ratio).

- Given the overall average, use the equation:

$$

0.4 \times 5x + 0.6 \times 4x = 55

$$

- Simplifying gives:

$$

2x + 2.4x = 55 \\

4.4x = 55 \\

x = \frac{55}{4.4} = 12.5

$$

- Therefore, the average weight of boys (5x) is:

$$

5 \times 12.5 = 62.5\, \text{kg}

$$

- 62.5 is option 1.