1 gallon is about 3.8 liters. 1 mile is about 1.6km.

35 mi/gal = (35 * 1.6km)/gal = (35 * 1.6km)/3.8l

You want liters per kilometer tho, so reciprocal: 3.8l/(1.6km * 35)

To drive 100 km, you need 100 times the fuel => (3.8l*100)/(1.6km * 35) ~ 6.8 l/100km