The mean and variance of five observations are 4.4 and 8.24 respectively. If three of these are 1, 2 and 6, find the other two observations.
Given: Mean of 5 observations = 4.4
and Variance of 5 observations = 8.24
Let the other two observations be x and y
∴, our observations are 1, 2, 6, x and y
Now, we know that,
⇒ 5 × 4.4 = 9 + x + y
⇒ 22 – 9= x + y
⇒ 13 = x + y
or x + y = 13 …(i)
Also,
Variance = 8.24
So,
⇒ 41.2 = 19.88 + (x2 + 19.36 – 8.8x) + (y2 + 19.36 – 8.8y)
⇒ 41.2 – 19.88 = x2 + 19.36 – 8.8x + y2 + 19.36 – 8.8y
⇒ 21.32 = x2 + y2 + 38.72 – 8.8(x + y)
⇒ x2 + y2 + 38.72 – 8.8(13) – 21.32 = 0 [from (i)]
⇒ x2 + y2 + 17.4 – 114.4 = 0
⇒ x2 + y2 – 97 = 0
⇒ x2 + y2 = 97 …(ii)
From eq. (i)
x + y = 17.4
Squaring both the sides, we get
(x + y)2 = (13)2
⇒ x2 + y2 + 2xy = 169
⇒ 97 + 2xy = 169 [from (ii)]
⇒ 2xy = 169 – 97
⇒ 2xy = 72
⇒ xy = 36
…(iii)
Putting the value of x in eq. (i), we get
x + y = 13
⇒ y2 + 36 = 13y
⇒ y2 – 13y + 36 = 0
⇒ y2 – 4y – 9y + 36 = 0
⇒ y(y – 4) – 9(y – 4)= 0
⇒ (y – 4)(y – 9) = 0
⇒ y – 4 = 0 and y – 9 = 0
⇒ y = 4 and y = 9
For y = 4
Hence, x = 9, y = 4 are the remaining two observations
For y = 9
Hence, x = 4, y = 9 are the remaining two observations
Thus, remaining two observations are 4 and 9.