Remember that the "mean" is the same as the average of a set of numbers.
To calculate the mean, we add all of the numbers together, and then divide that number, but the amount of numbers that we added together.
(Mean) = (Sum of all numbers) / (Amount of numbers)
First, let's look at the list of numbers given to us before we add the 15. We have,
11,5,8,22,19
Let's verify the mean of this list is actually 13,
Sum of all numbers = 11 + 5 + 8 + 22 + 19 = 65
Amount of numbers = 5
Mean = 65 / 5 = 13
So, it looks like the mean is actually 13, like the problem stated. Now let's add the 15 to the set, and do the same thing.
Sum of all numbers = 11 + 5 + 8 + 22 + 19 + 15 = 80
Amount of numbers = 6
Mean = 80 / 6
We can plug 80 / 6 into a calculator to get a decimal number of 13.333...
Rounding that to the nearest tenth, we get 13.3
