# In general, if you have m items of one type and n things of another type, how many possible ways can you choose one of each?

If there are $m$ items of Type A
then there are $m$ ways you can choose 1 item of Type A.
For each of these ways, if there are $n$ items of Type B,
then there are $n$ ways of choosing 1 item of Type B for each way of choosing an item of type A.
There are $m \times n$ ways of choosing one item of each type.