Suppose you have a data set that includes the values, 20, 30 and 50. if you add a fourth value to this data set, and after doing that the mean of the data set is 50, what was the value that you added?
a. -10
b. 40
c. 50 or
d. 100

To solve this question what you would need to do, is use the general equation for mean calculation and solve for the missing value. Knowing that the mean or the average is the sum of all values and or observations divided by the total number of observations noted we can figure out what the missing value is, using the formula.

x = unknown value
Mean = (20 + 30 + 50 + x)/4 = 50
Mean = (x + 100)/4 = 50
(x + 100)/4 = 50
(x + 100) = 50 * 4
x + 100 = 200
x = 200 - 100
x = 100

The unknown or the value that you had added was 100. 

The answer to this question is c