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Correct Answer: 50

Given that, total number of students = 150 = (n1+n2) and mean weight of 150 students = 60 kg. The mean weight of boys a1 = 70 kg and the mean weight of girls a2 = 55 kg Let n1 and n2 be the number of boys and girl, respectively.

Therefore, n1+n2 = 150 …(i)

11n1+11n2 = 1650 …(ii)

Average weight = n1a1+n2a2n1+n2

=> 60=70n1+55n2150

=> 70n1+55n2 = 9000

=> 14n1+11n2=1800 …..(iii)

On subtracting Eq. (ii) from Eq. (iii). we get

3n1=150=>n1=50

Hence, required number of boys is 50