An object at the surface of the earth (thus, a distance r from the center of the earth) weighs 180 n. its weight at a distance 3r from the center of the earth is:

20N

Explanation:

According to Newton's law of gravitation which states that every particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers

F = Gm1m2/r^2

Where

F = force between the masses

G universal gravitational constant

m1 and m2 = mass of the two particles

r = distance between the centre of the two mass

Therefore, weigh of an object on earth is inversely proportional to the square of its distance from the centre of the earth

W₁/W₂ = r₂²/r₁²

W₂ = W₁r₁²/r₂²

W₁ = 180 N

r₁ = R

r₂ = 3R

W₂ = (180 × R²)/(3R)²

W₂ = 180R²/9R²

W₂ = 180/9

W₂ = 20N

its weight at 3R is 20N