1. **State the problem:** Crystal skied 18 times and spent a total of 558. We need to find the cost per skiing trip. 2. **Formula used:** To find the cost per trip, use the formula: $$\text{Cost per trip} = \frac{\text{Total cost}}{\text{Number of trips}}$$ 3. **Apply the values:** $$\text{Cost per trip} = \frac{558}{18}$$ 4. **Simplify the fraction:** $$\frac{558}{18} = \frac{\cancel{18}31}{\cancel{18}1} = 31$$ 5. **Interpretation:** Each skiing trip cost Crystal 31. **Final answer:** 31