A sports car uses 8 litres of petrol for every 50 km it travels when its average driving speed is 85 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 sports car consume for a journey which lasts 15 h if it travels at 85 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 85 km/h, 8 ℓ → 50 km
At 60 km/h, 6 ℓ → 75 km

If 5 h at 85 km/h
Distance travelled = 5 x 85 = 425 km

At 85 km/h,
Amount of petrol used to travel 50 km = 8 ℓ
Amount of petrol used to travel 1 km = 8 ÷ 50 = 0.16 ℓ
Amount of petrol used to travel 425 km = 425 x 0.16 = 68 ℓ

Duration that the sports car travels for the rest of the journey
= 15 - 5
= 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 75 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 75 = 0.1 ℓ
Amount of petrol used to travel 600 km = 600 x 0.1 = 60 ℓ

Total petrol that the car will consume
= 68 + 60
= 128 ℓ

Answer(s): 128 ℓ