1. **State the problem:** Marco's dinner cost 100. We need to find the total cost including a 6% sales tax and a 15% tip on the original 100. 2. **Formula for sales tax and tip:** - Sales tax amount = original cost \times sales tax rate - Tip amount = original cost \times tip rate - Total cost = original cost + sales tax amount + tip amount 3. **Calculate sales tax:** $$\text{Sales tax} = 100 \times 0.06 = 6$$ 4. **Calculate tip:** $$\text{Tip} = 100 \times 0.15 = 15$$ 5. **Calculate total cost:** $$\text{Total cost} = 100 + 6 + 15 = 121$$ 6. **Answer:** The total cost of Marco's meal is **121**.