Question

At what distance above the surface of the earth is the acceleration due to the earth's gravity 0.785 m/s2 if the acceleration due to gravity at the surface has magnitude 9.80m/s2?

Answer 1

The acceleration due to gravity falls off as the inverse of the square of the distance from the center of the Earth. Thus the distance of interest is
1/(d/R)^2 = 0.785/9.8
d = R·√(9.8/0.785) ≈ R·3.53328
or R·2.53328 above the surface of the earth.

Wikipedia says the radius of the earth is 6371 km, so your distance is
(6371 km)·2.53328 ≈ 16,140 km