1. **State the problem:** Darius fills 18 gallons of gas for 45.60. We want to find the cost for Ben to fill 21 gallons. 2. **Formula:** Cost is proportional to gallons. Use unit rate: $$\text{Unit cost} = \frac{\text{Total cost}}{\text{Gallons}}$$ 3. **Calculate unit cost:** $$\text{Unit cost} = \frac{45.60}{18}$$ 4. **Simplify the fraction:** $$\frac{\cancel{45.60}}{\cancel{18}} = 2.5333...$$ So, cost per gallon is approximately $2.5333$. 5. **Calculate cost for 21 gallons:** $$\text{Cost} = 21 \times 2.5333 = 53.1993$$ 6. **Final answer:** Ben will pay approximately $53.20$ to fill 21 gallons.