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'Rugby and Ballet' printed from https://nrich.maths.org/
The mass of the $10$ ballet dancers is $$10 \times 40 \; \text{kg}= 400\; \text{kg} $$
The average mass of all $15$ people is $$\frac{425 \; \text{kg}+ 400 \; \text{kg}}{15}= \frac{825 \; \text{kg}}{15 }= 55\; \text{kg} $$