Correct Option: A

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.
∴ n₁ + n₂ = 150 ...(i)
11 n₁ + 11 n₂ = 1650 ...(ii)
Average weight = (n₁ a₁ + n₂ a₂) / (n₁ + n₂)
⇒ 60 = (70 n₁ + 55n₂)/150
⇒ 70 n₁ + 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.