Watch these videos and take notes. In the first problem, two objects (cyclists, in this case) are traveling in opposite directions toward each other. Note how Dr. Sousa uses the table to organize all the information given in the problem.
In the second problem, there is only one traveler, but she travels to another city and then back. Again, a table is useful to organize all the information given. Note that the task here is to find the distance, but the variable chosen to be denoted, x, is the time it takes to complete a one-way trip. You will find that this is a convenient approach in most problems where time is not given directly.
In the third example, two people travel in the same direction and one has to catch up to the other. Note that the key information you need to set up the equation is not given in the problem explicitly: both people will have traveled the same distance by the time one overtakes the other.