Let's look at the 3 sandwiches and 3 soups first and then expand the calculation. There are 9 ways I can have one of the sandwiches and 1 of the soups:
⎛
⎜
⎜
⎜
⎜
⎝
0
Soup 1
Soup 2
Soup 3
Sandwich 1
1
2
3
Sandwich 2
4
5
6
Sandwich 3
7
8
9
⎞
⎟
⎟
⎟
⎟
⎠
And so we can see that we multiply the number of sandwiches and the number of soups to get the total number of ways to get one of each.
The same works for more categories of choices, and so we multiply the 3 sandwiches, the 3 soups, 4 salads, and 2 drinks to get:
