A bus 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 60 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 6 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 → 60 km

If 6 h at 85 km/h
Distance travelled = 6 x 85 = 510 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 510 km = 510 x 0.2 = 102 ℓ

Duration that the bus travels for the rest of the journey
= 9 - 6
= 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 60 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 60 = 0.1 ℓ
Amount of petrol used to travel 180 km = 180 x 0.1 = 18 ℓ

Total petrol that the car will consume
= 102 + 18
= 120 ℓ

Answer(s): 120 ℓ