The cost price of 27 articles is the same as the selling price of 18 articles. What is the gain or loss percentage?

Let's assume the cost price and selling price of each article is CP and SP.
The cost price of 27 articles is the same as the selling price of 18 articles.

27CP = 18SP

3CP = 2SP

$$\frac{CP}{SP}\ =\ \frac{2}{3}$$

Let's assume CP = 2y and SP = 3y.

Here SP is more than CP. So the Gain percentage = $$\frac{\left(SP-CP\right)}{CP}\times100$$

= $$\frac{\left(3y-2y\right)}{2y}\times100$$

= $$\frac{y}{2y}\times100$$

= 50%