A lorry uses 8 litres of petrol for every 40 km it travels when its average driving speed is 85 km/h. It uses 6 litres of petrol for every 75 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 85 km/h for the first 7 hours and 60 km/h for the rest of the journey?
At 85 km/h, 8 ℓ → 40 km
At 60 km/h, 6 ℓ → 75 km

If 7 h at 85 km/h
Distance travelled = 7 x 85 = 595 km

At 85 km/h,
Amount of petrol used to travel 40 km = 8 ℓ
Amount of petrol used to travel 1 km = 8 ÷ 40 = 0.2 ℓ
Amount of petrol used to travel 595 km = 595 x 0.2 = 119 ℓ

Duration that the lorry travels for the rest of the journey
= 12 - 7
= 5 h

If 5 h at 60 km/h
Distance travelled = 5 x 60 = 300 km

At 60 km/h,
Amount of petrol used to travel 75 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 75 = 0.1 ℓ
Amount of petrol used to travel 300 km = 300 x 0.1 = 30 ℓ

Total petrol that the car will consume
= 119 + 30
= 149 ℓ

Answer(s): 149 ℓ