Weight = mass x acceleration due to gravity
The formula for calculating acceleration due to gravity is expressed as
acceleration due to gravity = GMe/Re^2
where
G is the gravitational constant
Me is the mass of the earth
Re is the radius of the earth
From the information given,
W = 720N
This means that
720 = mass x GMe/Re^2
Let the new weight be W1. The mass would remain constant because it is the quantity of matter in the body. It does not vary. At this new weight, the earth is two times as massive as it is and its radius were six times its present value. This means that
W1 = mass x G x 2Me/6^2Re^2 = mass x G x 2Me/36Re^2
W1 = mass x G x Me/18Re^2
By crossmultiplying both equations, we have
720 x mass x G x Me/18Re^2 = W1 x mass x GMe/Re^2
mass x G x Me/Re^2 cancels out on both sides of the equation. It becomes
W1 = 720/18
W1 = 40
Your new weight would be 40N