A sports car uses 11 litres of petrol for every 55 km it travels when its average driving speed is 90 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 sports car consume for a journey which lasts 8 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, 11 ℓ → 55 km
At 60 km/h, 6 ℓ → 80 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 55 km = 11 ℓ
Amount of petrol used to travel 1 km = 11 ÷ 55 = 0.2 ℓ
Amount of petrol used to travel 360 km = 360 x 0.2 = 72 ℓ

Duration that the sports car travels for the rest of the journey
= 8 - 4
= 4 h

If 4 h at 60 km/h
Distance travelled = 4 x 60 = 240 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 240 km = 240 x 0.1 = 24 ℓ

Total petrol that the car will consume
= 72 + 24
= 96 ℓ

Answer(s): 96 ℓ