Question

38. The mean weight of 150 students in a
class is 60 kg. The mean weight of boys
is 70 kg and that of girls is 55 kg. What
is the number of boys in the class?
[CDS 2012]
(a) 50
(b) 60
(c) 75
(d) 100​

Answer 1

Step-by-step explanation:

Mean weightof 150 students are 60 kg in a class

Mean weight of boys are 70 kg

And Girl are 55 kg

So no. of boys are Approx 50

Answer 2

Answer:

Let the number of boys and girls in the class be n₁ and n₂ respectively. Then

n₁ + n₂ = 150 — (i)

We have

x₁ = Mean weight of boy = 70 kgs

x₂ = Mean weight of girls = 55 kgs

x = Mean weight of all students = 60 kgs

∴ x = (n₁ x₁ + xn₂) / (n₁ + n₂)

⇒ 60 = (n₁ × 70 + n₂ × 55)/ (n₁ + n₂)

⇒ 60 (n₁ + n₂) = 70n₁ + 55n₂

⇒ 60n₁ + 60n₂ = 70n₁ + 55n₂

⇒ 10n₁ = 5n₂

⇒ 2n₁ = n₂ — (ii)

• Putting n₂ = 2n₁ from (ii) in (i), we get

⇒ n₁ + 2n₁ = 150

⇒ 3n₁ =150

⇒ n₁ = 50

• Putting n₁ = 50 in n₂ = 2n₁ we get

⇒ n₂ = 100

∴ 50 boys & 100 girls in the class.