Hard to explain, but here is an example:

In my first column, there are 3 options e.g.fireman, policeman and teacher.
In the second column are also three options e.g. 0-5 years, 5-10 years, over
10 years.

If I survey 20 people, and want to count how many firemen have worked for
over 10 years, how do I work out the formula?