A van uses 12 litres of petrol for every 40 km it travels when its average driving speed is 70 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 van consume for a journey which lasts 9 h if it travels at 70 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 70 km/h, 12 ℓ → 40 km
At 60 km/h, 6 ℓ → 75 km

If 5 h at 70 km/h
Distance travelled = 5 x 70 = 350 km

At 70 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 350 km = 350 x 0.3 = 105 ℓ

Duration that the van travels for the rest of the journey
= 9 - 5
= 4 h

If 4 h at 60 km/h
Distance travelled = 4 x 60 = 240 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 240 km = 240 x 0.1 = 24 ℓ

Total petrol that the car will consume
= 105 + 24
= 129 ℓ

Answer(s): 129 ℓ