Q. 33.5( 2 Votes )
The mean of first 8 observation is 18; last 8 observation is 20. If the mean of all 15 observation is 19 find the 8th observation
Answer :
Let 8th observation be x
Mean of first 8 observations = 18
where K is sum of first 7 terms
x = 144 – k ..(i)
Mean of last 8 observations = 20
Where L is sum of last 7 terms
x = 160 – L ..(ii)
Equating both the equations
160 – L = 144 – K
16 + K = L ..(iii)
Mean of all 15 terms = 19
K + X + L = 285
From equation (i), (ii), (iii)
K + (144-K) + (16 + K) = 285
⇒ K = 125
Replacing value in equation (i)
We get x = 19
So, 8th term is 19
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