Question

If the mean of the observation
x, x+3, x+5, x+7, x+10 is 9, the mean of the last three observation

Answer 1

mean = sum of all observations/total no. of observations
here, mean= x+x+3+x+5+x+7+x+10/5
9 = 5x+25/5
5x+25 = 45
5x = 20
x = 4
so we have data as follows:
4, 7, 9, 1,9

Answer 2

x+ x+3+x+5+x+7+x+10 = 45
 so, x = 4

So, mean of last three observations = 4+5+4+7+4+10 = 34/3