If the mean of five observations X, X-1, X-2, X-3 and x-4 is 20, then the mean of the first two observations is:

Correct option is C

​Given: Mean of five observations X, X-1, X-2, X-3, and X-4 = 20

Formula Used: The mean of the first two observations = (Sum of the first two observations) / 2

Solution:

(X + (X-1) + (X-2) + (X-3) + (X-4))/5 = 20
5X - 10 = 100
X = 22
Now, the mean of the first two observations is (22 + 21)/2 = 21.5.