A convertible uses 11 litres of petrol for every 55 km it travels when its average driving speed is 79 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 convertible consume for a journey which lasts 12 h if it travels at 79 km/h for the first 5 hours and 60 km/h for the rest of the journey?
At 79 km/h, 11 ℓ → 55 km
At 60 km/h, 6 ℓ → 60 km

If 5 h at 79 km/h
Distance travelled = 5 x 79 = 395 km

At 79 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 395 km = 395 x 0.2 = 79 ℓ

Duration that the convertible 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 60 km = 6 ℓ
Amount of petrol used to travel 1 km = 6 ÷ 60 = 0.1 ℓ
Amount of petrol used to travel 420 km = 420 x 0.1 = 42 ℓ

Total petrol that the car will consume
= 79 + 42
= 121 ℓ

Answer(s): 121 ℓ