Answer
$$40mi/gal$$
Work Step by Step
1. Use the following conversion factors: $1 km = 0.62137 mi$ and $1 L = 0.26417Gallon$
2. Multiply km value with conversion factor to miles, and liter with conversion factor to gallons.
$$(17 km\times0.62137miles/km)\div(1L\times0.26417 gallons/L) = 39.98671 mi/gal \approx 40mi/gal$$