A sports car uses 12 litres of petrol for every 40 km it travels when its average driving speed is 74 km/h. It uses 6 litres of petrol for every 65 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 10 h if it travels at 74 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 74 km/h, 12 ℓ → 40 km
At 60 km/h, 6 ℓ → 65 km

If 5 h at 74 km/h
Distance travelled = 5 x 74 = 370 km

At 74 km/h,
Amount of petrol used to travel 40 km = 12 ℓ
Amount of petrol used to travel 1 km = 12 ÷ 40 = 0.3 ℓ
Amount of petrol used to travel 370 km = 370 x 0.3 = 111 ℓ

Duration that the sports car travels for the rest of the journey
= 10 - 5
= 5 h

If 5 h at 60 km/h
Distance travelled = 5 x 60 = 300 km

At 60 km/h,
Amount of petrol used to travel 65 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 65 = 0.1 ℓ
Amount of petrol used to travel 300 km = 300 x 0.1 = 30 ℓ

Total petrol that the car will consume
= 111 + 30
= 141 ℓ

Answer(s): 141 ℓ