The question asks to find the total number of students at Ridgewood Junior High School. Here's the important information from the problem:

• 20% of students brought their lunch.
• 39 students brought their lunch.

We can set up a proportion to find the total number of students in the school.

Let $x$ be the total number of students. Since 20% (or $\frac{20}{100}$) of the total students equals 39 students, the equation is:

$\frac{20}{100} \times x = 39$

Now, solve for $x$.

First, multiply both sides by 100 to get rid of the fraction:

$20x = 39 \times 100$

$20x = 3900$

Next, divide both sides by 20:

$x = \frac{3900}{20} = 195$

So, there are 195 students in total at Ridgewood Junior High School.

Would you like a more detailed explanation or help with something else? Here are 5 related questions you might find useful:

1. How do you solve for percentages in other types of problems?
2. What happens when the percentage is unknown, but you know the total and a part?
3. Can you find how many students didn’t bring their lunch?
4. How would the problem change if 25% of the students brought lunch instead?
5. How can you use a different method, such as ratio reasoning, to solve this?

Tip: When solving percentage problems, always check if converting the percentage to a fraction helps simplify the equation!