Question

The mean weight of 150 students in a class is 60 kg. The mean weight of 50 boys of the class is 70kg, find the mean weight of the 100 girls of the class

Answer 1

Answer:

55 kg

Step-by-step explanation:

Let total sum of weight of boys be 'x'

x/50 = 70

x = 70 × 50

x is 3500 kgs

Let sum of weight of entire class be 'x'

x/150 = 60

x = 60 × 150

x is 9000 kgs

Weight of girls = 9000 - 3500 = 5500 kgs

Mean weight of girls

5500/100 = 55 kgs.

So mean weight of 100 girls o he class is 55 kgs

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.