Answer:

If a menu serves 5 people and requires 3 cups of flour, we can use proportion to find out how many cups of flour are needed for the menu to serve 20 people.

Let "x" be the number of cups of flour needed for the menu to serve 20 people. Then, we can set up the following proportion:

3 cups of flour / 5 people = x cups of flour / 20 people

To solve for "x", we can cross-multiply and simplify:

3 cups of flour x 20 people = 5 people x x cups of flour

60 cups of flour = 5x

x = 60 cups of flour / 5

x = 12 cups of flour

Therefore, if the menu serves 20 people, it will require 12 cups of flour.