A bus uses 10 litres of petrol for every 40 km it travels when its average driving speed is 84 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 12 h if it travels at 84 km/h for the first 8 hours and 60 km/h for the rest of the journey?
At 84 km/h, 10 ℓ → 40 km
At 60 km/h, 6 ℓ → 60 km

If 8 h at 84 km/h
Distance travelled = 8 x 84 = 672 km

At 84 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 672 km = 672 x 0.25 = 168 ℓ

Duration that the bus travels for the rest of the journey
= 12 - 8
= 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 60 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 60 = 0.1 ℓ
Amount of petrol used to travel 240 km = 240 x 0.1 = 24 ℓ

Total petrol that the car will consume
= 168 + 24
= 192 ℓ

Answer(s): 192 ℓ