In a recent issue of the IEEE Spectrum, 84 engineering conferences were announced. Four conferences lasted two days. Thirty-six lasted three days. Eighteen lasted four days. Nineteen lasted five days. Four lasted six days. One lasted seven days. One lasted eight days. One lasted nine days. Let X = the length (in days) of an engineering conference.

The data is shown in the table below.


Number of Conferences

Days Lasted

4

2

36

3

18

4

19

5

4

6

1

7

1

8

1

9



Calculate the sample mean of days of engineering conferences.