What must be the mass of a girl if she weighs 850 N on the earth? (Consider g = 9.8 m/s²)

The weight of the body is given as: 
Weight (W) = Mass (m) x Acceleration due to gravity (g)

Given,
W = 850 N

W = mg 

W = 850 N = m x 9.8 m/s²

$\Rightarrow$$\mathrm{m} = \frac{850 \mathrm{kg}.\mathrm{m}/{\mathrm{s}}^2}{9.8 \mathrm{m}/{\mathrm{s}}^2}$

$\mathrm{m} =86.73 \mathrm{kg}$

Therefore, the mass of the girl must be 86.73 kg.