A sports car uses 10 litres of petrol for every 50 km it travels when its average driving speed is 81 km/h. It uses 6 litres of petrol for every 85 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 13 h if it travels at 81 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 81 km/h, 10 ℓ → 50 km
At 60 km/h, 6 ℓ → 85 km

If 5 h at 81 km/h
Distance travelled = 5 x 81 = 405 km

At 81 km/h,
Amount of petrol used to travel 50 km = 10 ℓ
Amount of petrol used to travel 1 km = 10 ÷ 50 = 0.2 ℓ
Amount of petrol used to travel 405 km = 405 x 0.2 = 81 ℓ

Duration that the sports car travels for the rest of the journey
= 13 - 5
= 8 h

If 8 h at 60 km/h
Distance travelled = 8 x 60 = 480 km

At 60 km/h,
Amount of petrol used to travel 85 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 85 = 0.1 ℓ
Amount of petrol used to travel 480 km = 480 x 0.1 = 48 ℓ

Total petrol that the car will consume
= 81 + 48
= 129 ℓ

Answer(s): 129 ℓ