A class teacher has the following absentee record of 40 students of a class for the whole term. Find the mean number of days a student was absent.\begin{array}{|c|c|c|c|c|c|c|c|} \hline \text{Number of days} & 0 - 6 & 6 - 10 & 10 - 14 & 14 - 20 & 20 - 28 & 28 - 38 & 38 - 40 \\ \hline \text{Number of students} & 11 & 10 & 7 & 4 & 4 & 3 & 1 \\ \hline \end{array}