The mean of a data set is equal to the addition of the values of the data divided by the total number of values in the set.
In our problem:
![\begin{gathered} \operatorname{mean}=(13.5+16+12.6+15.2+12.8+11.8+17.2+12.1)/(8)=(111.2)/(8) \\ \Rightarrow\operatorname{mean}=13.9 \end{gathered}]()
Although you already calculated that, as I understand from what you wrote in the question tab.
The median of a data set can be found by ordering the values in the data set from least to greatest and then taking the middle value. In our problem:
![\begin{gathered} 11.8,12.1,12.6,12.8,13.5,15.2,16,17.2 \\ \Rightarrow\operatorname{median}=(12.8+13.5)/(2)=13.15 \\ \Rightarrow\operatorname{median}=13.15 \end{gathered}]()
How to divide 111.2 by 8:
The difference between Sarah's time and the mean time of the runners is:
![\operatorname{mean}-\text{Sarah}=13.9-12.1=1.8]()
The answer we are looking for is 1.8