A convertible uses 10 litres of petrol for every 50 km it travels when its average driving speed is 85 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 convertible consume for a journey which lasts 11 h if it travels at 85 km/h for the first 8 hours and 60 km/h for the rest of the journey?
At 85 km/h, 10 ℓ → 50 km
At 60 km/h, 6 ℓ → 70 km

If 8 h at 85 km/h
Distance travelled = 8 x 85 = 680 km

At 85 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 680 km = 680 x 0.2 = 136 ℓ

Duration that the convertible travels for the rest of the journey
= 11 - 8
= 3 h

If 3 h at 60 km/h
Distance travelled = 3 x 60 = 180 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 180 km = 180 x 0.1 = 18 ℓ

Total petrol that the car will consume
= 136 + 18
= 154 ℓ

Answer(s): 154 ℓ