You use 3 cans of soup to serve 5 people. if you want to serve 20 people, what should you do?

If you would like to know how many cans of soup you will need, you can calculate this using the following steps:

3 cans of soup ... 5 people
x cans of soup = ? ... 20 people

3 * 20 = 5 * x    /5
x = 3 * 20 / 5
x = 12 cans of soup

The correct result would be 12 cans of soup.

Most of the information’s that are required are already given in the question.
The number of cans used to serve 5 people = 3
Then
The number of cans required to serve 1 people = (3/5) * 1
                                                                             = (3/5) cans
So
The number of cans required to serve 20 people = (3/5) * 20 cans
                                                                               = 3 * 4 cans
                                                                               = 12 cans
So 12 cans will be required to serve 20 people. I hope the procedure is clear enough for you to understand.