A car uses 10 litres of petrol for every 40 km it travels when its average driving speed is 88 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 car consume for a journey which lasts 9 h if it travels at 88 km/h for the first 6 hours and 60 km/h for the rest of the journey?
At 88 km/h, 10 ℓ → 40 km
At 60 km/h, 6 ℓ → 65 km

If 6 h at 88 km/h
Distance travelled = 6 x 88 = 528 km

At 88 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 528 km = 528 x 0.25 = 132 ℓ

Duration that the car 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 65 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 65 = 0.1 ℓ
Amount of petrol used to travel 180 km = 180 x 0.1 = 18 ℓ

Total petrol that the car will consume
= 132 + 18
= 150 ℓ

Answer(s): 150 ℓ