A car uses 12 litres of petrol for every 60 km it travels when its average driving speed is 83 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 car consume for a journey which lasts 12 h if it travels at 83 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 83 km/h, 12 ℓ → 60 km
At 60 km/h, 6 ℓ → 65 km

If 5 h at 83 km/h
Distance travelled = 5 x 83 = 415 km

At 83 km/h,
Amount of petrol used to travel 60 km = 12 ℓ
Amount of petrol used to travel 1 km = 12 ÷ 60 = 0.2 ℓ
Amount of petrol used to travel 415 km = 415 x 0.2 = 83 ℓ

Duration that the car travels for the rest of the journey
= 12 - 5
= 7 h

If 7 h at 60 km/h
Distance travelled = 7 x 60 = 420 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 420 km = 420 x 0.1 = 42 ℓ

Total petrol that the car will consume
= 83 + 42
= 125 ℓ

Answer(s): 125 ℓ