The mean of 5 observations x,x+2,x+4,x+6,x+8 is 11.
Mean=NumberofobservationsTotalobservations
11=x+x+2+x+4+x+6+x+85
55=5x+20
5x=35
x=7
Then, the first three terms will be,
x=7
x+2=7+2
=9
x+4=7+4
=11
The mean of first three terms is,
Mean=7+9+113
Mean=273
Mean=9