A lorry uses 11 litres of petrol for every 50 km it travels when its average driving speed is 75 km/h. It uses 6 litres of petrol for every 60 km it travels when its average driving speed is 60 km/h. How much petrol will the lorry consume for a journey which lasts 14 h if it travels at 75 km/h for the first 8 hours and 60 km/h for the rest of the journey?
At 75 km/h, 11 ℓ → 50 km
At 60 km/h, 6 ℓ → 60 km

If 8 h at 75 km/h
Distance travelled = 8 x 75 = 600 km

At 75 km/h,
Amount of petrol used to travel 50 km = 11 ℓ
Amount of petrol used to travel 1 km = 11 ÷ 50 = 0.22 ℓ
Amount of petrol used to travel 600 km = 600 x 0.22 = 132 ℓ

Duration that the lorry travels for the rest of the journey
= 14 - 8
= 6 h

If 6 h at 60 km/h
Distance travelled = 6 x 60 = 360 km

At 60 km/h,
Amount of petrol used to travel 60 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 60 = 0.1 ℓ
Amount of petrol used to travel 360 km = 360 x 0.1 = 36 ℓ

Total petrol that the car will consume
= 132 + 36
= 168 ℓ

Answer(s): 168 ℓ