1. **State the problem:** Roy's mass decreased from 80 kg to 72 kg due to illness. We need to find the percentage decrease in mass. 2. **Formula for percentage decrease:** $$\text{Percentage decrease} = \left(\frac{\text{Original mass} - \text{New mass}}{\text{Original mass}}\right) \times 100$$ 3. **Substitute the values:** $$\text{Percentage decrease} = \left(\frac{80 - 72}{80}\right) \times 100$$ 4. **Calculate the difference:** $$80 - 72 = 8$$ 5. **Simplify the fraction:** $$\frac{8}{80} = \frac{\cancel{8}}{\cancel{80}} = \frac{1}{10}$$ 6. **Calculate the percentage:** $$\frac{1}{10} \times 100 = 10$$ 7. **Final answer:** The mass decreased by **10%**.