Explanation:

- Determining cost price and selling price:

- Cost price (CP) of 27 articles = Selling price (SP) of 18 articles

- Using CP and SP to find gain/loss:

- Let's assume CP of 1 article = x, then CP of 27 articles = 27x

- Therefore, SP of 18 articles = 27x

- So, SP of 1 article = 27x/18 = 3x/2

- Finding gain percentage:

- Gain = SP per article - CP per article = (3x/2 - x) = x/2

- Gain percentage = (Gain/CP) * 100 = (x/2x) * 100 = 50%

- Explaining options:

- Option 1: 50% gain - This reflects our derived gain percentage correctly.

- Option 2: 60% loss - This is incorrect based on calculations.

- Option 3: 30% loss - This is incorrect as calculations show gain, not loss.

- Option 4: 90% gain - There is gain, but not to this extent.

- Correct Answer: 50% gain. Your answer is correct.