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.