A convertible uses 8 litres of petrol for every 50 km it travels when its average driving speed is 75 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 convertible consume for a journey which lasts 12 h if it travels at 75 km/h for the first 7 hours and 60 km/h for the rest of the journey?
At 75 km/h, 8 ℓ → 50 km
At 60 km/h, 6 ℓ → 80 km

If 7 h at 75 km/h
Distance travelled = 7 x 75 = 525 km

At 75 km/h,
Amount of petrol used to travel 50 km = 8 ℓ
Amount of petrol used to travel 1 km = 8 ÷ 50 = 0.16 ℓ
Amount of petrol used to travel 525 km = 525 x 0.16 = 84 ℓ

Duration that the convertible travels for the rest of the journey
= 12 - 7
= 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 80 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 80 = 0.1 ℓ
Amount of petrol used to travel 300 km = 300 x 0.1 = 30 ℓ

Total petrol that the car will consume
= 84 + 30
= 114 ℓ

Answer(s): 114 ℓ