A sports car uses 10 litres of petrol for every 40 km it travels when its average driving speed is 90 km/h. It uses 6 litres of petrol for every 60 km it travels when its average driving speed is 60 km/h. How much petrol will the sports car consume for a journey which lasts 14 h if it travels at 90 km/h for the first 4 hours and 60 km/h for the rest of the journey?
At 90 km/h, 10 ℓ → 40 km
At 60 km/h, 6 ℓ → 60 km

If 4 h at 90 km/h
Distance travelled = 4 x 90 = 360 km

At 90 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 360 km = 360 x 0.25 = 90 ℓ

Duration that the sports car travels for the rest of the journey
= 14 - 4
= 10 h

If 10 h at 60 km/h
Distance travelled = 10 x 60 = 600 km

At 60 km/h,
Amount of petrol used to travel 60 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 60 = 0.1 ℓ
Amount of petrol used to travel 600 km = 600 x 0.1 = 60 ℓ

Total petrol that the car will consume
= 90 + 60
= 150 ℓ

Answer(s): 150 ℓ