Q3 of 69 Page 6

The mean weight of 21 students of a class is 52 kg. If the mean weight of the first 11 students of the class is 50 kg and that of the last 11 students is 54 kg, find the weight of the 11th student.

Let the weight of 11th student be x


Given that mean weight of 21 students = 52kg


∴ sum of 21 students weight = 21 × 52 = 1092kg


Mean weight of the first 11 students = 50kg


Sum of first 11 students weight = 11 × 50 = 550kg


Mean weight of the last 11 students = 54kg


Sum of the last 11 students weight = 11 × 54 = 594kg


Therefore,


Sum of first 11 students weight + sum of last 11 students weight – weight of the 11th student = sum of 21 students weight


550+594 - x = 1092


⇒ 1144 – x = 1092


⇒ x = 52


Hence, weight of 11th student is 52kg


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