# The mean of first 8 observation is 18 & last 8observation is 20 . if thevmean of all 15 observation is 19 find the 8th observation

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by divya10

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by divya10

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Then sum of first 8 obs = 18×8 = 144

Mean of last 8 obs = 20

Then sum of last 8 obs = 20×8 = 160

Let the 8th observation be x

Then 144 and 160 both have x

Sum of all 15 observations = 144+160 - x [as there are 2 x s one in 144 and the other in 160 only one x is present in total observations]

Mean of all 15 observations is given as 19

⇒ (144 +160 -x)/15 = 19

⇒ 304 - x = 19×15

⇒ 304 - x = 285

⇒ -x = 285 -304

⇒ -x = -19

∴ x = 19

∴ The 8th observation is 19