A lorry uses 9 litres of petrol for every 45 km it travels when its average driving speed is 88 km/h. It uses 6 litres of petrol for every 80 km it travels when its average driving speed is 60 km/h. How much petrol will the lorry consume for a journey which lasts 11 h if it travels at 88 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 88 km/h, 9 ℓ → 45 km
At 60 km/h, 6 ℓ → 80 km

If 5 h at 88 km/h
Distance travelled = 5 x 88 = 440 km

At 88 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 440 km = 440 x 0.2 = 88 ℓ

Duration that the lorry travels for the rest of the journey
= 11 - 5
= 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 80 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 80 = 0.1 ℓ
Amount of petrol used to travel 360 km = 360 x 0.1 = 36 ℓ

Total petrol that the car will consume
= 88 + 36
= 124 ℓ

Answer(s): 124 ℓ