Aracing car used 255 litres of fuel to complete
a 340 km race.
on average, how many litres of fuel did the car
use every 100 km

To answer thie question, you must first develop a proportional statement that compares the liters of fuel used to the distance driven. Considering the subject matter of your question, said statement should be formatted like this:

(255L/340km)=(x/100)

Since this is a proportional statement, you can scale up or down the relative unit to complete the statement. In this case, you will be solving for liters, represented by x, so solve the statement using a ratio for the other side of the equation, in this case the denominators:

340/100

You should get  3.4. That is what you will be scaling the other unit with. Now it is a simple matter to substitute for the numerators:

255/3.4=x

After solving this equation, your final answer should be x=75.

Hope that helps.