Answer :
With the position of the girl as a reference point, the center of mass is 8 m from the girl position.
If there are two objects as shown on the attached picture, the center of mass is:
xc = m₁ / (m₁ + m₂) . x₁ + m₂ / (m₁ + m₂) . x₂
From the problem, we know that the boy and his skis have twice the inertia of the girl and her skis. We can replace the mass in the equation with inertia, hence:
xc = I₁ / (I₁ + I₂) . x₁ + I₂ / (I₁ + I₂) . x₂
Let:
I₁ = the girl's inertia
I₂ = the boy's inertia
Hence,
I₂ = 2 I₁
Let make the position of the girl as a reference point, then,
x₁ = 0
x₂ = 12
Plug these parameters into the formula:
xc = I₁ / (I₁ + I₂) . 0 + 2 I₁ / (I₁ + 2 I₁) . 12
xc = 2/3 . 12 = 8 m
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