Question

Mark was making himself a sandwich. He could choose white or wheat bread. He could choose turkey, chicken, tuna, or peanut butter. How many different combinations of sandwiches could Mark make?

Answer 1

You can choose 1 type of bread out of 2 AND
You can choose 1 type of protein out of 4

So you find the number of ways to do both by multiplying the number of ways to do the 1st times the number of ways to do the 2nd:

$$
\binom{2}{1}\binom{4}{1}
$$

That's it!

http://en.wikipedia.org/wiki/Combination

Answer 2

I suppose another way to think about it is:
For white bread: you have 4 different options of "fillings"
For wheat bread: you have the same 4 options.

So, altogether, you have 4 + 4 = 8 different combinations.