Question

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?

Answer 1

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 `mxxn` ways of choosing one item of each type.