A convertible uses 9 litres of petrol for every 45 km it travels when its average driving speed is 71 km/h. It uses 6 litres of petrol for every 90 km it travels when its average driving speed is 60 km/h. How much petrol will the convertible consume for a journey which lasts 15 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, 9 ℓ → 45 km
At 60 km/h, 6 ℓ → 90 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 45 km = 9 ℓ
Amount of petrol used to travel 1 km = 9 ÷ 45 = 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
= 15 - 5
= 10 h

If 10 h at 60 km/h
Distance travelled = 10 x 60 = 600 km

At 60 km/h,
Amount of petrol used to travel 90 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 90 = 0.1 ℓ
Amount of petrol used to travel 600 km = 600 x 0.1 = 60 ℓ

Total petrol that the car will consume
= 71 + 60
= 131 ℓ

Answer(s): 131 ℓ