Final answer:
To calculate the petrol needed for 120km, use the consumption rate (5 litres for 80km), which gives 1 litre/16km. Multiplying this rate by 120km results in 7.5 litres needed.
Step-by-step explanation:
The student is asking how many litres of petrol would be needed to travel 120km, given that 5 litres are used to travel 80km at a constant speed. This question involves working with ratios and can be solved using a simple proportion.
First, we find the petrol consumption rate per kilometre by dividing the volume of petrol used by the distance travelled:
Consumption Rate = 5 litres / 80 km
This simplifies to:
Consumption Rate = 1 litre / 16 km
Next, we use this rate to calculate the amount of petrol needed for 120km:
Litres per 120km = (1 litre / 16 km) × 120 km
Litres per 120km = 7.5 litres
Therefore, the car would need 7.5 litres of petrol to travel 120km at the same speed.