1. **State the problem:** Johnny Boy starts with 100 aura and loses 50 aura after tripping on a rock. 2. **Formula used:** To find the remaining aura, subtract the lost aura from the initial aura. $$\text{Remaining aura} = \text{Initial aura} - \text{Lost aura}$$ 3. **Apply the values:** $$\text{Remaining aura} = 100 - 50$$ 4. **Calculate:** $$\text{Remaining aura} = 50$$ 5. **Conclusion:** Johnny Boy now has 50 aura left after losing 50 aura.