We are given the ratio $3:1$ and need to find the number that makes the ratio equivalent to $36 : x$.

To solve this, we can set up a proportion:

$\frac{3}{1} = \frac{36}{x}$

Now, cross-multiply:

$3 \cdot x = 1 \cdot 36$

This simplifies to:

$3x = 36$

Now, divide both sides by 3:

$x = \frac{36}{3} = 12$

So, the number $x$ that makes the ratio equivalent is 12.

Would you like more details or have any questions?

Here are 5 related questions to expand on this:

1. How do you simplify a ratio?
2. What is the process for solving a proportion?
3. Can you apply the same method to solve $5:2$ and $50:x$?
4. How would you solve if the ratio was reversed, such as $1:3$?
5. What real-world scenarios involve equivalent ratios?

Tip: When solving proportions, always ensure the ratios are written in the same order for consistency.