The given observations are x, x + 3, x + 5, x + 7, x + 10.
Mean of the given observations = 9
Number of observations = 5
Now,
∴ x + 5 = 4 + 5 = 9
x + 7 = 4 + 7 = 11
x + 10 = 4 + 10 = 14
So, the last three observations are 9, 11 and 14.
∴ Mean of the last three observations =
Thus, the mean of the last three observations is .
Hence, the correct answer is option (c).