The radius of the moon is one eighty-first that of the earth. If the acceleration due to gravity on the surface of the earth is 9.8m/s^2 , What is its value on the moon's surface.

2 answers

(9.8m/s^2)/6 = 1.63 m/s^2.
I guess you are to assume the density rho is the same
g = G M/R^2
if M = rho V = rho (4/3) pi R^3
then
g = G rho (4/3) pi R^3/R^2 = constant * R if the density is constant
that would say g earth is 81 times that of the moon
HOWEVER as Henry pointed out it is SIX times
in other words the earth is far less dense than the moon.
NOW WITHOUT YOUR TYPING ERROR!!!!!!
Mass of moon = Mass of earth / 81
Radius of moon = Radius of earth / 4
gearth = G M/R^2
gmoon = G M/81 / R^3/16
gearth/gmoon = 81/16 = 5.06 or close to what Henry told you (6)