If the mean of five observations x , x + 2 , x + 4 , x + 6 , x + 8  is 11, find the mean of first three observations.

Explanation

Observations given: `x,x+2,x+4,x+6,x+8`
Sum of observations=`x+(x+2)+(x+4)+(x+6)+(x+8) = 5x+20= 5(x+4)`
Mean=Sum of observations/No. of observations

`=(5(x+4))/5=11`
`x+4=11`
`therefore x=7`
So, the observations are: `7,9,11,13,15`
Required mean `=(7+9+11)/3=9`
Hence, the required answer is `9`