Probability problems typically give you a table with values. Your first step is to get row and column totals. In some problems, you will be given totals and need to calculate a few missing values in the table.
Your next step is to decide if the problem is asking about the entire population (in which case you use the grand total "n").
Some problems are asking about a subset of the population, these are conditional probability problems. What is the one word that tells you that you have a conditional probability problem?