A car uses 12 litres of petrol for every 60 km it travels when its average driving speed is 75 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 car consume for a journey which lasts 9 h if it travels at 75 km/h for the first 4 hours and 60 km/h for the rest of the journey?
At 75 km/h, 12 ℓ → 60 km
At 60 km/h, 6 ℓ → 80 km

If 4 h at 75 km/h
Distance travelled = 4 x 75 = 300 km

At 75 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 300 km = 300 x 0.2 = 60 ℓ

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

If 5 h at 60 km/h
Distance travelled = 5 x 60 = 300 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 300 km = 300 x 0.1 = 30 ℓ

Total petrol that the car will consume
= 60 + 30
= 90 ℓ

Answer(s): 90 ℓ