I teach this and some might call me sick, but I have always enjoyed mean, median, and mode.
Mode is the most frequent number. (Remember that mode and most have the same 2 letters at the beginning and you'll remember it.) This might be used if you are a shoe sales clerk. Which pair of shoes sold the most? If there is a tie than all the numbers that tie for first are the mode. Example: in your situation 7,9, and 11 are the most frequent as they all happen 4 times. So the mode would be (7,9,11). If you think of shoes this makes sense. Wouldn't you want to know which pairs sells the most? Not having a mode is incredibly rare. This is when ALL numbers happen the same number of times like your 2nd sample.
The median is the middle number when you put the numbers from low to high. Only problem is when you have an even number of numbers to organize, there is no exact middle number. Therefore you take the 2 middle numbers and average them.
The mean and average are the same thing.
Any more questions, just ask. :)
