The mass of the $10$ ballet dancers is $$10 \times 40 \; \text{kg}= 400\; \text{kg} $$

The average mass of all $15$ people is $$\frac{425 \; \text{kg}+ 400 \; \text{kg}}{15}= \frac{825 \; \text{kg}}{15 }= 55\; \text{kg} $$

*This problem is taken from the UKMT Mathematical Challenges.*