The five observations are x+1, x+2, x+3, x+4, and x+5.
Their mean is 15.
Mean=Number of observationsSum of all observations
15=5(x+1)+(x+2)+(x+3)+(x+4)+(x+5)
15=55x+15
75=5x+15
60=5x
x=12
The first three observations are:
x+1=12+1=13
x+2=12+2=14
x+3=12+3=15
Mean of first three observations=313+14+15
=342
=14
Therefore, the mean of the first three observations is 14.