To solve this, we can set up a proportion based on the given information. Here's the process:

1. Mrs. Thompson's recipe uses 3 cups of flour for 4 servings.
2. We need to find the number of cups of flour (let's call this $x$) needed for 20 servings.

We set up the proportion:

$\frac{3 \, \text{cups of flour}}{4 \, \text{servings}} = \frac{x \, \text{cups of flour}}{20 \, \text{servings}}$

Now, we solve for $x$:

$\frac{3}{4} = \frac{x}{20}$

Cross-multiply:

$3 \times 20 = 4 \times x$

$60 = 4x$

Now, solve for $x$:

$x = \frac{60}{4} = 15$

So, Mrs. Thompson will need 15 cups of flour for 20 servings.

Would you like any further explanation?

Here are 5 related questions:

1. How many cups of flour are needed for 10 servings?
2. How many servings can be made with 9 cups of flour?
3. If she increases the recipe to use 5 cups of flour, how many servings will that make?
4. What if she only has 12 cups of flour? How many servings can she make?
5. How much flour would be needed for 25 servings?

Tip: Setting up proportions is a helpful way to solve scaling problems in recipes and other real-life scenarios.