The mean is the sum of the data divided by the number of data points. The problem is the way that you represented your data. You wrote 80+83+79+86 which is really only one number. So if you really meant "What is the mean of 80, 83, 79, and 86, then sum the 4 numbers and divide by 4. But if you meant exactly what you wrote, only divide by 1.
John
My calculator said it, I believe it, that settles it
