A car returns 18 km per litre in city traffic and 24 km per litre on the highway. On a 360 km trip, 120 km of the running is in the city and the rest on the highway. With petrol at ₹105 a litre, the fuel bill for the trip (in ₹) is:

City: 120/18 = 20/3 litres. Highway: 240/24 = 10 litres. Together 50/3 litres, and $50/3\times 105$ = ₹1750. (Averaging the two mileages to 21 km per litre gives 360/21 ≈ 17.14 litres and ₹1800 — mileages, like speeds, cannot be averaged when the two stretches are unequal. Running the whole 360 km at 24 km/l gives ₹1575 and at 18 km/l gives ₹2100, the two extremes the true answer must sit between.)