A bus uses 9 litres of petrol for every 45 km it travels when its average driving speed is 85 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 bus consume for a journey which lasts 9 h if it travels at 85 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 85 km/h, 9 ℓ → 45 km
At 60 km/h, 6 ℓ → 80 km

If 5 h at 85 km/h
Distance travelled = 5 x 85 = 425 km

At 85 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 425 km = 425 x 0.2 = 85 ℓ

Duration that the bus travels for the rest of the journey
= 9 - 5
= 4 h

If 4 h at 60 km/h
Distance travelled = 4 x 60 = 240 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 240 km = 240 x 0.1 = 24 ℓ

Total petrol that the car will consume
= 85 + 24
= 109 ℓ

Answer(s): 109 ℓ