If the cost price of 10 articles is equal to the selling price of 9 articles, then the gain or loss percent is

Explanation

To solve the problem, we will follow these steps:

### Step 1: Define the Variables
Let the cost price (CP) of 1 article be \( x \) and the selling price (SP) of 1 article be \( y \).

### Step 2: Set Up the Equation
According to the question, the cost price of 10 articles is equal to the selling price of 9 articles. Therefore, we can write the equation:
\[
...