A van uses 10 litres of petrol for every 40 km it travels when its average driving speed is 79 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 van consume for a journey which lasts 14 h if it travels at 79 km/h for the first 4 hours and 60 km/h for the rest of the journey?
At 79 km/h, 10 ℓ → 40 km
At 60 km/h, 6 ℓ → 90 km

If 4 h at 79 km/h
Distance travelled = 4 x 79 = 316 km

At 79 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 316 km = 316 x 0.25 = 79 ℓ

Duration that the van travels for the rest of the journey
= 14 - 4
= 10 h

If 10 h at 60 km/h
Distance travelled = 10 x 60 = 600 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 600 km = 600 x 0.1 = 60 ℓ

Total petrol that the car will consume
= 79 + 60
= 139 ℓ

Answer(s): 139 ℓ