We have that the electric field at the center of the metal ball due only to the charges on the surface of the metal ball is

From the question we are told that
A solid metal ball of radius 1.5 cm
bearing a charge of -15 nC is located near a hollow plastic ball of radius 1.9 cm bearing
uniformly distributed charge of -7 nC
The distance between the centers of the balls is 9 cm
Generally the equation for the electric field is mathematically given as


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Answer: A little more that 5 Kg for a healthy person
Explanation: First, we know the following:
The regular adult has from 9 to 12 pints of blood. This is around 5 liters for a healthy male adult.
The human body is composed mostly on water, around 80%.
Blood is mostly composed on plasma, which makes blood thicker than water.
Knowing that, almost all the body is compose of water, it is safe to think that blood density should be near to that of water but higher.
The density on water is a know value. Which makes the following true:
<em>1 Liter of Water weights 1 Kg</em>
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It could be said then, that the total mass of blood for a healthy person should be a little more that 5 kgs.
The angular momentum of the person is given by:

where

is the mass of the person,

is the tangential velocity, and

is the distance of the person from the center of rotation. Using these data, we find
City 4 because it is closer to the equator