Fundamental Principle of Counting: 

If an event can occur in m different ways, and another event can occur in n different ways, then the total number of occurrences of the events is m * n.

Ex: Suppose we have 3 pairs of shoes and 4 pairs of socks. In how many ways can you wear them?

Solution:

We have 3 pairs of shoes and 4 pairs of socks.
So, we can wear it 3 × 4 ways
I.e., 12 ways