x = the missing value
The data set is {18, 16, 9, 12, 23, x}
There are n = 6 values in this set.
To get the mean, we add up the values.
Then divide by the sample size n = 6.
mean = (sum of values)/(number of values)
mean = (18+16+9+12+23+x)/6
mean = (78+x)/6
(78+x)/6 = 15
78+x = 6*15
78+x = 90
x = 90-78
x = 12 is the missing value
The mean of {18, 16, 9, 12, 23, 12} is 15.
Answer: 12