Answer:
mean = 20.75
Explanation:
the mean is calculated as
mean =

that is the sum of the data set ÷ number of values in data set
sum = 13 + 15 + 7(20) + 23 + 27 + 31
= 13 + 15 + 140 + 23 + 27 + 31
= 249
and count = 12
substitute these values into the formula for mean
mean =
= 20.75