A van uses 12 litres of petrol for every 60 km it travels when its average driving speed is 90 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 van consume for a journey which lasts 15 h if it travels at 90 km/h for the first 8 hours and 60 km/h for the rest of the journey?
At 90 km/h, 12 ℓ → 60 km
At 60 km/h, 6 ℓ → 90 km

If 8 h at 90 km/h
Distance travelled = 8 x 90 = 720 km

At 90 km/h,
Amount of petrol used to travel 60 km = 12 ℓ
Amount of petrol used to travel 1 km = 12 ÷ 60 = 0.2 ℓ
Amount of petrol used to travel 720 km = 720 x 0.2 = 144 ℓ

Duration that the van travels for the rest of the journey
= 15 - 8
= 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 90 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 90 = 0.1 ℓ
Amount of petrol used to travel 420 km = 420 x 0.1 = 42 ℓ

Total petrol that the car will consume
= 144 + 42
= 186 ℓ

Answer(s): 186 ℓ