Consider a data consisting of 10 observations x1,x2, ,x10, whose mean is 5 and variance is 7. If the mean and the variance of the first 8 observations x1,x2, ,x8 are 4 and 3.5, respectively, and x9<x10, then the value of 3x9+2x10 is .
Answer
44
Solution
xi2=n( 2+ x2).
All 10: xi=50, xi2=10(7+25)=320. First 8: =32, xi2=8(3.5+16)=156.
x9+x10=18, x92+x102=164 x9x10= 324-1642=80.
t2-18t+80=0 t=8,10; with x9<x10: x9=8, x10=10.
3x9+2x10=24+20=44.
Answer: 44