A lorry uses 10 litres of petrol for every 40 km it travels when its average driving speed is 80 km/h. It uses 6 litres of petrol for every 70 km it travels when its average driving speed is 60 km/h. How much petrol will the lorry consume for a journey which lasts 13 h if it travels at 80 km/h for the first 6 hours and 60 km/h for the rest of the journey?
At 80 km/h, 10 ℓ → 40 km
At 60 km/h, 6 ℓ → 70 km

If 6 h at 80 km/h
Distance travelled = 6 x 80 = 480 km

At 80 km/h,
Amount of petrol used to travel 40 km = 10 ℓ
Amount of petrol used to travel 1 km = 10 ÷ 40 = 0.25 ℓ
Amount of petrol used to travel 480 km = 480 x 0.25 = 120 ℓ

Duration that the lorry travels for the rest of the journey
= 13 - 6
= 7 h

If 7 h at 60 km/h
Distance travelled = 7 x 60 = 420 km

At 60 km/h,
Amount of petrol used to travel 70 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 70 = 0.1 ℓ
Amount of petrol used to travel 420 km = 420 x 0.1 = 42 ℓ

Total petrol that the car will consume
= 120 + 42
= 162 ℓ

Answer(s): 162 ℓ