1. **State the problem:** We know that 10% of the students brought their lunch, and this corresponds to 40 students. We need to find the total number of students in the school. 2. **Formula used:** To find the total number of students when a percentage and its corresponding value are known, use the proportion: $$\frac{\text{part}}{\text{percent}} = \frac{\text{whole}}{100}$$ where "part" is the number of students who brought lunch, "percent" is 10, and "whole" is the total number of students. 3. **Set up the equation:** $$\frac{40}{10} = \frac{x}{100}$$ where $x$ is the total number of students. 4. **Solve for $x$ by cross-multiplying:** $$40 \times 100 = 10 \times x$$ $$4000 = 10x$$ 5. **Divide both sides by 10 to isolate $x$:** $$\frac{4000}{\cancel{10}} = \frac{10x}{\cancel{10}}$$ $$400 = x$$ 6. **Answer:** There are **400** students in total at Ridgewood Junior High School.