Answer: 26
Step-by-step explanation
x = missing value
To find the mean, we add up the values. Then divide by the sample size (which in this case is n = 7)
Adding up the values gives us
21+29+18+14+x+30+9 = x+121
Divide that over the sample size n = 7 to get the mean 21.
(x+121)/7 = 21
x+121 = 7*21
x+121 = 147
x = 147-121
x = 26 is the missing number