A convertible uses 10 litres of petrol for every 50 km it travels when its average driving speed is 71 km/h. It uses 6 litres of petrol for every 60 km it travels when its average driving speed is 60 km/h. How much petrol will the convertible consume for a journey which lasts 10 h if it travels at 71 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 71 km/h, 10 ℓ → 50 km
At 60 km/h, 6 ℓ → 60 km

If 5 h at 71 km/h
Distance travelled = 5 x 71 = 355 km

At 71 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 355 km = 355 x 0.2 = 71 ℓ

Duration that the convertible 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 60 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 60 = 0.1 ℓ
Amount of petrol used to travel 300 km = 300 x 0.1 = 30 ℓ

Total petrol that the car will consume
= 71 + 30
= 101 ℓ

Answer(s): 101 ℓ