10 is the mean of a set of 7 observations and 5 is the mean of a set of 2 observations, the mean of the combined set is
15
10
8.5
none of these
We have
n1=7,X¯1=10,n2=3 and X¯2=5
Let X be the Mean of the combined set. Then,
X¯=n1X¯1+n2X¯2m1+n2=7×10+3×57+3=8.5