Answer:
F0 / 81
Step-by-step explanation:
Let the two charges by Q and q which are separated by d.
By use of coulomb's law
F0 = k Q q / d^2 ......(1)
Now the charges are decreased by factor of 9.
Q' = Q / 9
q' = q / 9 ......(2)
Now the Force is
F' = k Q' q' / d^2
F' = k (Q /9) (q / 9) / d^2
F' = k Q q / 81d^2
F' = F0 / 81