A bus uses 10 litres of petrol for every 40 km it travels when its average driving speed is 85 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 bus consume for a journey which lasts 9 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 → 40 km
At 60 km/h, 6 ℓ → 65 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 40 km = 10 ℓ
Amount of petrol used to travel 1 km = 10 ÷ 40 = 0.25 ℓ
Amount of petrol used to travel 680 km = 680 x 0.25 = 170 ℓ

Duration that the bus travels for the rest of the journey
= 9 - 8
= 1 h

If 1 h at 60 km/h
Distance travelled = 1 x 60 = 60 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 60 km = 60 x 0.1 = 6 ℓ

Total petrol that the car will consume
= 170 + 6
= 176 ℓ

Answer(s): 176 ℓ