It looks to me like you all you need to remember is what mean, median and mode are.
The mean is just what you would call the average. You take the sum of all the numbers in your data set and divide them by how many numbers there are. You need that to come out to 15. So, for example, if your set were 13, 14, 15, 16, 17 your mean would be 15.
Your median would also be 15 here because it is the number that is the halfway point of all the numbers in the data set when they are put in order.
A mode is the number that occurs most often in the data set. So your set would have to have an equal number of 13s and 15s and no other number that occurred that many times.
Follow the link for more explanation of these terms.