Q2 of 69 Page 6

The mean of 17 observations is 20. If the mean of the first 9 observation is 23 and that of last 9 observations is 18, find the 9th observation.

Let the 9th observation be x


Given that mean of 17 observations = 20


∴ sum of 17 observations = 17 × 20 = 340


Mean of the first 9 observations= 23


Sum of first 9 observations = 9 × 23 = 207


Mean of the last 9 observations = 18


Sum of the last 9 observations = 9 × 18 = 162


Therefore,


Sum of first 9 observations + sum of last 9 observations – 9th observation = sum of 17 observations


207 + 162 - x = 340


⇒ 369 – x = 340


⇒ x = 29


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