Apostrophes are used to indicate: the original point the new point

In order to the get the mean, you will have to add all of the values and then divide your answer by how many values there are. 

Let's add our values: 
78+79+80+80+89+89+92+93+94+95+95+95 = 1059. 

There are 12 values. So...:
1059/12 = 88.25. It says to round to the nearest tenth. That would be 88.3.