To convert gallons to liters, you can use the conversion factor that 1 gallon is approximately 3.78541 liters.
To convert 15 gallons to liters:
\[ 15 , \text{gallons} \times 3.78541 , \text{liters/gallon} \approx 56.78115 , \text{liters} \]
Therefore, 15 gallons is approximately 56.78 liters.