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Question:
The mean and variance of 7 observations are 8 and 16, respectively. If five of the observations are 2, 4, 10, 12, 14. Find the remaining two observations.
Answer:

Given mean and variance of 7 observations are 8 and 16.

Five observations are 2, 4, 10, 12, 14.

Let the other two observations are x and y.

So 7 observations are : 2, 4, 10, 12, 14 ,x ,y

Now

       Mean =  (2+4+10+12+14+x+y)/7

=> 8 = (2+4+10+12+14+x+y)/7

=> 8*7 = 2+4+10+12+14+x+y

=> 56 = 42 + x+ y

=> x + y = 56-42

=> x + y = 14 ...................1

Again Given varience = 16

=> (1/7)*∑ (xi - mean)2 = 16          (7<=i<=1)

=> ∑ (xi - mean)2 = 16*7

=> ∑ (xi - mean)2 = 112

=> {(2-8)2 +(4-8)2 + (10-8)2 + (12-8)2 + (14-8)2 + (x-8)2 + (y-8)2 } = 112

=> {(-6)2 +(-4)2 + (2)2 + (4)2 + (6)2 + x2 + 64 - 16x + y2 + 64 -16y } = 112

=> {36 + 16 + 4 + 16 + 36 + x2 + y2 + 64 + 64 -16(x+y) } = 112

=> {108 + x2 + y2 + 128 -(16*14)} = 112        (since x+y =14)

=> {108 + x2 + y2 + 128 -224} = 112

=> x2 + y2 + 236 - 224 = 112

=> x2 + y2 + 12 = 112

=> x2 + y2 = 12-12

=> x2 + y2 = 100................2

Squaring equation 1, we get

(x+y)2 = 196

=> x2 + y2 + 2xy = 196

=> 100 + 2xy = 196

=> 2xy = 196-100

=>2xy = 96

=> xy = 96/2

=> xy = 48.............3

Now (x-y)2 = x2 + y2 - 2xy

                  = 100 - 2*48

                  = 100-96

                  = 4

=> x-y = √2

=> x-y = 2, -2

case 1: when x-y = 2 and x+y = 14

After solving it, we get x = 8, y= 6

case 2: when x-y = -2 and x+y = 14

After solving it, we get x = 6, y= 8

So the two numbers are 6 and 8

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