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

If there are 5 people, they get 3 cans.

We want to give the same amount of soup to everyone, so if there are twice as many people, we're going to need twice as much soup.

(10 people, 6 cans)

Now say we had 20 people. (4 times as many)

We'd use 4 times as many cans soup, which would be 3*4 = 12 cans.