I did not understand this question based on ratio and proportion

Question

The mileage of a car is 16 km per litre, if the fuel is totally unadulterated. If due to adulteration the mileage reduces by 25%, by what percentage does the cost of maintaining the car per kilometre increase?( Assume that maintenance cost is equal to fuel cost)

When there is no adulteration then mileage is 16 km per litre but when there is adulteration then new mileage is $3/4*16=12$

$4/16*100=25$ But the answer is 33.33. How?

Answer 1

Driving 48 km previously took 3 liters of fuel; now it takes 4 liters.

The additional liter is one third of the previous cost.

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When you go from speaking about "distance per amount of fuel" to "fuel per fixed distances", the numbers -- and therefore also the ratios -- are replaced by their reciprocals.

So a decrease to $\frac34$ of the former number corresponds to an increase to $\frac43$ of the latter number.

$\frac34$ is $25\%$ less than $1$, but $\frac43$ is $33\frac13\%$ more than $1$.

Answer 2

Initially you pay $x$ for $16km$

Now we pay $x$ for $12km$, equivalently, you pay $\frac43x$ for $16km$.

Percentage of cost increase is $$\frac{\text{percentage of change}}{\text{initial amount}}=\frac{\frac43x-x}{x}=\frac13$$