Given that,
Initial force, F = 12 N
First initial charge, q₁ = 3C
First new charge, q₁' = 6C
Initial distance, r = 15 cm
New distance, r' = 45 cm
To find,
The new force of attraction.
Solution,
The force between two charges is given by :


Let F' is the new force.


As q₂ is same in this case.
Dividing equation (1) and (2) :

So, the new force of attraction is 2.67 N.