Fuel adulteration: a trader mixes “25% kerosene to petrol” and sells the mixture at the price of petrol. If kerosene costs 50% of petrol, find the net profit percentage (assume “25% to petrol” means kerosene equals 25% of petrol volume; i.e., 1 part kerosene mixed with 4 parts petrol → kerosene is 20% of the final mixture).

Introduction:
Ambiguous phrasing “mixes 25% kerosene to petrol” is commonly interpreted as adding kerosene equal to 25% of the petrol volume (not that the final mixture is 25% kerosene). Under that standard reading, kerosene becomes 1/5 of the final mixture.

Given Data / Assumptions:

• Let petrol price = P per litre; kerosene price = 0.5P per litre.
• Mixing rule: for 4 L petrol add 1 L kerosene ⇒ total 5 L mixture (20% kerosene).
• Mixture sold at price of petrol = P per litre.

Concept / Approach:
Compute cost per litre of the mixture from component costs, then compare to SP per litre (which equals P). Profit% = (SP − CP)/CP * 100.

Step-by-Step Solution:
Component cost for 5 L: 4L*P + 1L*0.5P = 4P + 0.5P = 4.5PCost per litre of mixture = 4.5P / 5 = 0.9PSP per litre = P ⇒ Profit per litre = P − 0.9P = 0.1PProfit% = 0.1P / 0.9P * 100 = 11.111...% = 11 1/9 %

Verification / Alternative check:
If instead the statement meant 25% of the final mixture is kerosene, profit would be 14 2/7%, which does not match the given options. Hence we adopt the standard interpretation producing 11 1/9 %.

Why Other Options Are Wrong:

• 12 1/9 % / 9 1/11 % / 20 %: not aligned with the cost-per-litre calculation under the stated assumption.

Common Pitfalls:

• Misreading “25% to petrol” as “25% of the mixture,” changing the ratio and answer.

Final Answer:
11 1/9 %