The rule of the mean is
Mean = sum/number
The given data is
30, 34, 32, 31, 34, 39, 36, 30, 32, 34, 38, 39, 35, 37, 38
There are 15 numbers, then
The number = 15
We will add all of them to find the sum
The sum = 519
Then divide it by 15 to find the mean
The mean is 34.6