1. **State the problem:** We need to find the total gallons of water used per day if 16.8% of the total is 67.2 gallons. 2. **Formula used:** To find the total when a percentage and part of the total are known, use: $$\text{Total} = \frac{\text{Part}}{\text{Percentage as a decimal}}$$ 3. **Convert percentage to decimal:** $$16.8\% = \frac{16.8}{100} = 0.168$$ 4. **Set up the equation:** $$0.168 \times \text{Total} = 67.2$$ 5. **Solve for Total:** $$\text{Total} = \frac{67.2}{0.168}$$ 6. **Show intermediate step with cancellation:** $$\text{Total} = \frac{\cancel{67.2}}{\cancel{0.168}}$$ 7. **Calculate:** $$\text{Total} = 400$$ 8. **Answer:** The family uses **400 gallons** of water per day in total.