A car uses 11 litres of petrol for every 50 km it travels when its average driving speed is 90 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 12 h if it travels at 90 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 90 km/h, 11 ℓ → 50 km
At 60 km/h, 6 ℓ → 65 km

If 5 h at 90 km/h
Distance travelled = 5 x 90 = 450 km

At 90 km/h,
Amount of petrol used to travel 50 km = 11 ℓ
Amount of petrol used to travel 1 km = 11 ÷ 50 = 0.22 ℓ
Amount of petrol used to travel 450 km = 450 x 0.22 = 99 ℓ

Duration that the car travels for the rest of the journey
= 12 - 5
= 7 h

If 7 h at 60 km/h
Distance travelled = 7 x 60 = 420 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 420 km = 420 x 0.1 = 42 ℓ

Total petrol that the car will consume
= 99 + 42
= 141 ℓ

Answer(s): 141 ℓ