A bus uses 10 litres of petrol for every 40 km it travels when its average driving speed is 78 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 12 h if it travels at 78 km/h for the first 4 hours and 60 km/h for the rest of the journey?
At 78 km/h, 10 ℓ → 40 km
At 60 km/h, 6 ℓ → 65 km

If 4 h at 78 km/h
Distance travelled = 4 x 78 = 312 km

At 78 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 312 km = 312 x 0.25 = 78 ℓ

Duration that the bus travels for the rest of the journey
= 12 - 4
= 8 h

If 8 h at 60 km/h
Distance travelled = 8 x 60 = 480 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 480 km = 480 x 0.1 = 48 ℓ

Total petrol that the car will consume
= 78 + 48
= 126 ℓ

Answer(s): 126 ℓ