A class has 100 students with an average weight of 60 kg. If there are 50 boys with an average weight of 70 kg, what is the average weight of the girls and are they heavier than boys?

There are 100 students with an average weight of 60 kg.

$$\text{Total class weight} = 100 \text{ students} \times 60 \text{ kg/student} = 6000 \text{ kg}$$

There are 50 boys with an average weight of 70 kg.

$$\text{Total boys' weight} = 50 \text{ boys} \times 70 \text{ kg/boy} = 3500 \text{ kg}$$

$$\text{Total girls' weight} = \text{Total class weight} - \text{Total boys' weight}$$ $$\text{Total girls' weight} = 6000 \text{ kg} - 3500 \text{ kg} = 2500 \text{ kg}$$

$$\text{Average girls' weight} = \frac{\text{Total girls' weight}}{\text{Number of girls}}$$ $$\text{Average girls' weight} = \frac{2500 \text{ kg}}{50 \text{ girls}} = 50 \text{ kg/girl}$$

The average weight of girls is 50 kg.
The average weight of boys is 70 kg.
Since $50 \text{ kg} < 70 \text{ kg}$, girls are not heavier than boys on average.

On average, girls are not heavier than boys. The average weight of girls is $\boxed{\text{50 kg}}$, while the average weight of boys is 70 kg.