A convertible uses 11 litres of petrol for every 55 km it travels when its average driving speed is 76 km/h. It uses 6 litres of petrol for every 90 km it travels when its average driving speed is 60 km/h. How much petrol will the convertible consume for a journey which lasts 9 h if it travels at 76 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 76 km/h, 11 ℓ → 55 km
At 60 km/h, 6 ℓ → 90 km

If 5 h at 76 km/h
Distance travelled = 5 x 76 = 380 km

At 76 km/h,
Amount of petrol used to travel 55 km = 11 ℓ
Amount of petrol used to travel 1 km = 11 ÷ 55 = 0.2 ℓ
Amount of petrol used to travel 380 km = 380 x 0.2 = 76 ℓ

Duration that the convertible travels for the rest of the journey
= 9 - 5
= 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 90 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 90 = 0.1 ℓ
Amount of petrol used to travel 240 km = 240 x 0.1 = 24 ℓ

Total petrol that the car will consume
= 76 + 24
= 100 ℓ

Answer(s): 100 ℓ