If a person weighs 180 pounds on the surface of the earth and the radius of the earth is 3900 miles, what will the person weigh if he or she is 850 miles above the earth's surface

Gravity is inversely proportional to the square of the distance.

In English:

Your weight at the Earth's surface and your weight 850 miles above the Earth's surface decreases by a factor of

(4,750 / 3,900) ^ 2 = 1.483399080

So your weight would decrease by (your weight / 1.483399080)

If you weigh 180 pounds, at 850 miles above the Earth your weight is

180 / 1.483399080 which equals

121.3429362515 pounds.