A lorry uses 12 litres of petrol for every 40 km it travels when its average driving speed is 84 km/h. It uses 6 litres of petrol for every 90 km it travels when its average driving speed is 60 km/h. How much petrol will the lorry consume for a journey which lasts 8 h if it travels at 84 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 84 km/h, 12 ℓ → 40 km
At 60 km/h, 6 ℓ → 90 km

If 5 h at 84 km/h
Distance travelled = 5 x 84 = 420 km

At 84 km/h,
Amount of petrol used to travel 40 km = 12 ℓ
Amount of petrol used to travel 1 km = 12 ÷ 40 = 0.3 ℓ
Amount of petrol used to travel 420 km = 420 x 0.3 = 126 ℓ

Duration that the lorry travels for the rest of the journey
= 8 - 5
= 3 h

If 3 h at 60 km/h
Distance travelled = 3 x 60 = 180 km

At 60 km/h,
Amount of petrol used to travel 90 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 90 = 0.1 ℓ
Amount of petrol used to travel 180 km = 180 x 0.1 = 18 ℓ

Total petrol that the car will consume
= 126 + 18
= 144 ℓ

Answer(s): 144 ℓ