A sports car uses 12 litres of petrol for every 50 km it travels when its average driving speed is 80 km/h. It uses 6 litres of petrol for every 70 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 11 h if it travels at 80 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 80 km/h, 12 ℓ → 50 km
At 60 km/h, 6 ℓ → 70 km

If 5 h at 80 km/h
Distance travelled = 5 x 80 = 400 km

At 80 km/h,
Amount of petrol used to travel 50 km = 12 ℓ
Amount of petrol used to travel 1 km = 12 ÷ 50 = 0.24 ℓ
Amount of petrol used to travel 400 km = 400 x 0.24 = 96 ℓ

Duration that the sports car travels for the rest of the journey
= 11 - 5
= 6 h

If 6 h at 60 km/h
Distance travelled = 6 x 60 = 360 km

At 60 km/h,
Amount of petrol used to travel 70 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 70 = 0.1 ℓ
Amount of petrol used to travel 360 km = 360 x 0.1 = 36 ℓ

Total petrol that the car will consume
= 96 + 36
= 132 ℓ

Answer(s): 132 ℓ