A lorry uses 9 litres of petrol for every 45 km it travels when its average driving speed is 70 km/h. It uses 6 litres of petrol for every 65 km it travels when its average driving speed is 60 km/h. How much petrol will the lorry consume for a journey which lasts 12 h if it travels at 70 km/h for the first 4 hours and 60 km/h for the rest of the journey?
At 70 km/h, 9 ℓ → 45 km
At 60 km/h, 6 ℓ → 65 km

If 4 h at 70 km/h
Distance travelled = 4 x 70 = 280 km

At 70 km/h,
Amount of petrol used to travel 45 km = 9 ℓ
Amount of petrol used to travel 1 km = 9 ÷ 45 = 0.2 ℓ
Amount of petrol used to travel 280 km = 280 x 0.2 = 56 ℓ

Duration that the lorry travels for the rest of the journey
= 12 - 4
= 8 h

If 8 h at 60 km/h
Distance travelled = 8 x 60 = 480 km

At 60 km/h,
Amount of petrol used to travel 65 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 65 = 0.1 ℓ
Amount of petrol used to travel 480 km = 480 x 0.1 = 48 ℓ

Total petrol that the car will consume
= 56 + 48
= 104 ℓ

Answer(s): 104 ℓ