The depth d at which the value of acceleration due to gravity becomes 1n times the value at the surface, is [R =radius of the earth]
Rn
R(n-1n)
Rn2
R(nn+1)
g' = g(1-dR) ⇒gn = g(1-dR)
⇒dR = 1-1n ⇒d = (n-1n)R