Radius of the earth is 6400km and g is 9.8 m÷s find the value of g produced on a mass of 20 kg at a distance of 10000km from the earths surface ?

first, g is m/s^2, not m/s

F = GmM/r^2 = 9.8m

you have a new R=100/64 r = 25/16 r
So, g will be (16/25)^2 as big

9.8 * (16/25)^2 = 4.01 m/s^2