Question

if r inversely proportional to the fifth root of s and r=8 when s=125 find s when r=3

Answer 1

r = k (1/s^(1/5))
when r = 8, s = 125
8 = k (1/125(1/5))
was expecting a nice answer, are you sure you did not want the third root?
But anyway ...
8 = k(.38073....)
k = 21.0122.. -----> r = 21.0122..(1/s^(1/5))

when r = 3
3 = 21.0122(1/s^(1/5)
s^(1/5) = 7.00407..
s = 16,855.967